Improve navigation between main app, conditions, and dashboard

- Main app: split tagline into attribution + Station/Dashboard nav pills (always visible, styled with border/hover)
- Conditions page: add Dashboard → link and ← App back-link in header
- Dashboard: add Conditions link, rename App → ← Weather in header nav

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/public/index.html

@@ -29,7 +29,11 @@
       <button class="tab" data-tab="learn">Learn</button>
       <button class="tab" data-tab="settings">Settings</button>
     </nav>
-    <span class="tagline">Open-Meteo · RainViewer · <a href="/conditions" class="tagline-link">Station</a> · <a href="/dashboard" class="tagline-link">Dashboard</a></span>
+    <span class="tagline">Open-Meteo · RainViewer</span>
+    <nav class="station-links">
+      <a href="/conditions" class="station-link">Station</a>
+      <a href="/dashboard" class="station-link">Dashboard</a>
+    </nav>
   </header>
 
   <!-- Loading -->