add FrostyColors ThemeExtension, align hardcoded colors with M3 roles

Introduce FrostyColors ThemeExtension with 7 semantic color fields
(highlightedMessage, emoteOnly, followersOnly, slowMode, subOnly,
uniqueMode, overlayOnSurface) and brightness-aware light/dark variants.

Replace hardcoded colors across 15 files:
- Links use colorScheme.primary instead of Colors.blue
- Mentions use colorScheme.error instead of Colors.red
- First-time chatters share highlightedMessage (Twitch purple)
- Chat mode indicators use frostyColors fields
- Overlay text uses frostyColors.overlayOnSurface

Remove Provider<FrostyThemes> pattern from main.dart, channel.dart,
video_overlay.dart, and frosty_photo_view_dialog.dart.

Replace manual onSurface.withValues(alpha: 0.6) with M3's built-in
onSurfaceVariant for secondary text in section headers, chat modes,
alerts, loading indicators, and input hints. Fix double-muted
onSurfaceVariant.withValues(alpha: 0.6) in navigation icons and
notification dismiss button.

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

lib/widgets/loading_indicator.dart

@@ -19,9 +19,7 @@ class LoadingIndicator extends StatelessWidget {
         Text(
           subtitle!,
           style: TextStyle(
-            color: Theme.of(
-              context,
-            ).colorScheme.onSurface.withValues(alpha: 0.5),
+            color: Theme.of(context).colorScheme.onSurfaceVariant,
           ),
         ),
       ],