Show 'Link Discord' only when Discord linking is configured (#213)

The account menu offered 'Link Discord' to every signed-in user even on a
server with no Discord OAuth app configured, where it dead-ends (the start route
303-bounces to /alerts). Add GET /api/v2/discord/config reporting whether linking
is enabled, and have the menu render the entry only when it is. On a server
without Discord set up nothing surfaces; setting the OAuth env vars later makes
the entry appear on its own — no code change needed to turn it on.

static/account.js

@@ -6,6 +6,8 @@
 // every page the way units.js is (imported by each page's entry module).
 
 let currentUser = null;          // {id, email, display_name} or null
+let discordEnabled = false;      // is Discord linking configured on the server?
+let discordChecked = false;      // have we asked yet? (once per page load)
 const authCbs = [];              // notified on login/logout so pages can re-gate
 
 // The app's base path (e.g. "/thermograph"), derived from this module's own URL
@@ -59,6 +61,16 @@ async function refreshUser() {
     const res = await apiFetch("api/v2/users/me");
     currentUser = res.ok ? await res.json() : null;
   } catch (e) { currentUser = null; }
+  // Learn once whether Discord linking is configured, so the account menu only
+  // offers "Link Discord" when it will actually work (checked lazily, and only
+  // for a signed-in user — the menu never shows it to anyone else).
+  if (currentUser && !discordChecked) {
+    discordChecked = true;
+    try {
+      const r = await apiFetch("api/v2/discord/config");
+      if (r.ok) discordEnabled = (await r.json()).enabled === true;
+    } catch (e) { /* leave it hidden */ }
+  }
   return currentUser;
 }
 
@@ -304,7 +316,7 @@ function renderHeader() {
           ${currentUser.discord_id
             ? `<button type="button" class="acct-pop-link acct-discord-dm">${currentUser.discord_dm ? "Discord alerts: on" : "Discord alerts: off"}</button>`
               + '<button type="button" class="acct-pop-link acct-discord-unlink">Unlink Discord</button>'
-            : `<a href="${APP_BASE}/api/v2/discord/link/start" class="acct-pop-link">Link Discord</a>`}
+            : (discordEnabled ? `<a href="${APP_BASE}/api/v2/discord/link/start" class="acct-pop-link">Link Discord</a>` : "")}
           <button type="button" class="acct-pop-link acct-signout">Sign out</button>
         </div>
       </div>`;