Add an Alerts page to the nav with a "what are alerts" explainer (#153)

Put Alerts in the main navigation (after Climate) across the interactive pages
and the SEO/climate pages, so it's a discoverable entry point rather than only the
header bell. The bell stays; this is an additional way in.

The signed-out /alerts gate now leads with a clear "What are weather alerts?"
explainer — what they do, how the percentile threshold works, and an example —
above the sign-in / create-account button, so visitors understand the feature
before signing in.

The extra nav item pushed the inline header past ~800px into a horizontal
scrollbar; trim the tab padding in the 641–1000px band so the row fits again
(phones already collapse into the hamburger).

templates/base.html.j2

@@ -41,6 +41,7 @@
         <a href="{{ base }}/day">Day Detail</a>
         <a href="{{ base }}/compare">Compare</a>
         <a href="{{ base }}/climate"{% if section == 'climate' %} class="active"{% endif %}>Climate</a>
+        <a href="{{ base }}/alerts">Alerts</a>
       </nav>
         </div>
       </details>