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>

wwwroot/js/app.js

@@ -8,7 +8,12 @@ window.sptb = {
         document.body.removeChild(link);
     },
     setTheme: function(theme) {
-        document.documentElement.setAttribute('data-theme', theme);
+        var t = (theme || 'System').toLowerCase();
+        if (t === 'system') {
+            t = (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches)
+                ? 'dark' : 'light';
+        }
+        document.documentElement.setAttribute('data-theme', t);
     },
     scrollToBottom: function(el) {
         if (el) el.scrollTop = el.scrollHeight;