Fix GUI polish issues across auth modal, theme, and 404

- Add missing modal CSS (.modal-overlay/.modal-dialog/.modal-header):
  the "Connect to Microsoft" auth modal was rendering unstyled inline
  at the bottom of the page. Now a centered dialog with backdrop.
- Surface OAuth connect errors in the modal instead of silently
  reopening it with no explanation.
- MainLayout: implement IDisposable so event handlers are actually
  unsubscribed (Dispose existed but was never invoked).
- Wire up the Settings theme selector (was a dead control): drop the
  unsupported Dark option, call sptb.setTheme on save and on load,
  resolve System via prefers-color-scheme.
- Add branded 404 page via UseStatusCodePagesWithReExecute + Routes
  <NotFound> (blank white page before).
- Add .progress-fill.indeterminate animation and .progress-panel.
- Home: replace inline JS hover handlers with a .feature-card CSS class.
- Define missing --surface-2 variable referenced by MainLayout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Components/Pages/Home.razor

@@ -29,8 +29,8 @@ else
     <div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(200px,1fr));gap:16px;margin-top:16px">
         @foreach (var feature in _features)
         {
-            <a href="@feature.Href" style="text-decoration:none">
-                <div class="card" style="cursor:pointer;transition:box-shadow .15s" onmouseover="this.style.boxShadow='0 2px 8px rgba(0,120,212,.2)'" onmouseout="this.style.boxShadow=''">
+            <a href="@feature.Href" style="text-decoration:none;color:inherit">
+                <div class="card feature-card">
                     <div style="font-size:28px;margin-bottom:8px">@feature.Icon</div>
                     <div style="font-weight:600;margin-bottom:4px">@feature.Title</div>
                     <div class="text-muted">@feature.Description</div>