Link previews: Open Graph tags + logo for shared URLs (#35)

Sharing a Thermograph URL (Discord, Slack, iMessage…) now unfurls into a
card with the site name, page title, description, accent color, and logo.

- All five pages get description/theme-color/og:*/twitter:card meta and a
  favicon. og:url/og:image need absolute URLs and crawlers don't run JS, so
  pages carry an __ORIGIN__ placeholder the server fills in per request from
  X-Forwarded-Proto (Caddy) / scheme + the Host header + base path — correct
  on both the LAN dev server and the prod domain without hardcoding either.
- New logo assets: the header's ▚ mark as an app icon (accent quadrants on
  the dark surface) — logo.png (512x512, the og:image) and logo.svg (favicon).
- Page routes render the substitution with a weak ETag + 304 revalidation
  (replacing plain FileResponse) and now answer HEAD, which preview crawlers
  probe with (previously 404).

static/calendar.html

@@ -4,6 +4,23 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <title>Thermograph — 2-year calendar</title>
+  <meta name="description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta name="theme-color" content="#f0803c" />
+  <!-- Link previews (Discord, Slack, iMessage…). The server fills the og:url /
+       og:image origins in from this request's scheme://host + base path —
+       crawlers need absolute URLs and don't run JS. -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Thermograph" />
+  <meta property="og:title" content="Thermograph — 2-year calendar" />
+  <meta property="og:description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta property="og:url" content="__ORIGIN__/calendar" />
+  <meta property="og:image" content="__ORIGIN__/logo.png" />
+  <meta property="og:image:width" content="512" />
+  <meta property="og:image:height" content="512" />
+  <meta property="og:image:alt" content="Thermograph logo" />
+  <meta name="twitter:card" content="summary" />
+  <link rel="icon" href="logo.svg" type="image/svg+xml" />
+  <link rel="apple-touch-icon" href="logo.png" />
   <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" />
   <link rel="stylesheet" href="style.css" />
 </head>

static/compare.html

@@ -4,6 +4,23 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <title>Thermograph — compare comfort</title>
+  <meta name="description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta name="theme-color" content="#f0803c" />
+  <!-- Link previews (Discord, Slack, iMessage…). The server fills the og:url /
+       og:image origins in from this request's scheme://host + base path —
+       crawlers need absolute URLs and don't run JS. -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Thermograph" />
+  <meta property="og:title" content="Thermograph — compare comfort" />
+  <meta property="og:description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta property="og:url" content="__ORIGIN__/compare" />
+  <meta property="og:image" content="__ORIGIN__/logo.png" />
+  <meta property="og:image:width" content="512" />
+  <meta property="og:image:height" content="512" />
+  <meta property="og:image:alt" content="Thermograph logo" />
+  <meta name="twitter:card" content="summary" />
+  <link rel="icon" href="logo.svg" type="image/svg+xml" />
+  <link rel="apple-touch-icon" href="logo.png" />
   <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" />
   <link rel="stylesheet" href="style.css" />
 </head>

static/day.html

@@ -4,6 +4,23 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <title>Thermograph — a single day</title>
+  <meta name="description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta name="theme-color" content="#f0803c" />
+  <!-- Link previews (Discord, Slack, iMessage…). The server fills the og:url /
+       og:image origins in from this request's scheme://host + base path —
+       crawlers need absolute URLs and don't run JS. -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Thermograph" />
+  <meta property="og:title" content="Thermograph — a single day" />
+  <meta property="og:description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta property="og:url" content="__ORIGIN__/day" />
+  <meta property="og:image" content="__ORIGIN__/logo.png" />
+  <meta property="og:image:width" content="512" />
+  <meta property="og:image:height" content="512" />
+  <meta property="og:image:alt" content="Thermograph logo" />
+  <meta name="twitter:card" content="summary" />
+  <link rel="icon" href="logo.svg" type="image/svg+xml" />
+  <link rel="apple-touch-icon" href="logo.png" />
   <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" />
   <link rel="stylesheet" href="style.css" />
 </head>

static/index.html

@@ -4,6 +4,23 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <title>Thermograph — how unusual is your weather?</title>
+  <meta name="description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta name="theme-color" content="#f0803c" />
+  <!-- Link previews (Discord, Slack, iMessage…). The server fills the og:url /
+       og:image origins in from this request's scheme://host + base path —
+       crawlers need absolute URLs and don't run JS. -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Thermograph" />
+  <meta property="og:title" content="Thermograph — how unusual is your weather?" />
+  <meta property="og:description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta property="og:url" content="__ORIGIN__/" />
+  <meta property="og:image" content="__ORIGIN__/logo.png" />
+  <meta property="og:image:width" content="512" />
+  <meta property="og:image:height" content="512" />
+  <meta property="og:image:alt" content="Thermograph logo" />
+  <meta name="twitter:card" content="summary" />
+  <link rel="icon" href="logo.svg" type="image/svg+xml" />
+  <link rel="apple-touch-icon" href="logo.png" />
   <link rel="stylesheet" href="https://unpkg.com/leaflet@1.9.4/dist/leaflet.css" />
   <link rel="stylesheet" href="style.css" />
 </head>

static/legend.html

@@ -4,6 +4,23 @@
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <title>Thermograph — reading the grades</title>
+  <meta name="description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta name="theme-color" content="#f0803c" />
+  <!-- Link previews (Discord, Slack, iMessage…). The server fills the og:url /
+       og:image origins in from this request's scheme://host + base path —
+       crawlers need absolute URLs and don't run JS. -->
+  <meta property="og:type" content="website" />
+  <meta property="og:site_name" content="Thermograph" />
+  <meta property="og:title" content="Thermograph — reading the grades" />
+  <meta property="og:description" content="Pick any point on Earth and see how its recent weather stacks up against ~45 years of local climate history — graded days, calendars, forecasts, and side-by-side comparisons." />
+  <meta property="og:url" content="__ORIGIN__/legend" />
+  <meta property="og:image" content="__ORIGIN__/logo.png" />
+  <meta property="og:image:width" content="512" />
+  <meta property="og:image:height" content="512" />
+  <meta property="og:image:alt" content="Thermograph logo" />
+  <meta name="twitter:card" content="summary" />
+  <link rel="icon" href="logo.svg" type="image/svg+xml" />
+  <link rel="apple-touch-icon" href="logo.png" />
   <link rel="stylesheet" href="style.css" />
 </head>
 <body>

static/logo.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <!-- The ▚ brand mark from the header, as an app icon: two accent quadrants on
+       the app's dark surface. Keep in sync with logo.png (the og:image raster). -->
+  <rect width="512" height="512" rx="100" fill="#171b21"/>
+  <rect x="116" y="116" width="134" height="134" rx="20" fill="#f0803c"/>
+  <rect x="262" y="262" width="134" height="134" rx="20" fill="#f0803c"/>
+</svg>