Introduces candy-gloss tile rendering (diagonal gradient + glass highlight
+ optional glow) via a shared paintGlossyTile() in tile_painter.dart,
applied to board filled-cells and tray/drag-overlay pieces. Adds
ThemeColors to palette.dart for UI-layer season color resolution, and
activeThemeProvider for one-call access to the active season's theme.
Regenerates the game_screen golden to reflect the new look.
Pure-Dart WAV synthesizer (tool/gen_sfx.dart) generates place/clear/
combo/win/lose effects; AudioService player pool fires on placement,
line clears, combo streaks, and phase transitions.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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