feat: dashboard manifest supports 'supported_games' field, frontend filters by selected game

- DashboardTheme model adds supported_games (default: 'all')
- Built-in dashboards declare game compatibility:
  speedometer/tachometer/gear_indicator -> all
  lap_timer -> forza_horizon_5,forza_horizon_4,forza_motorsport,f1_24
- Dashboard browser filters themes by selected game
- Card shows '全' (all) or '限' (limited) badge
- Updated DASHBOARD_DEV.md with supported_games spec

dashboards/speedometer/manifest.json

@@ -5,10 +5,10 @@
     "description": "大字号实时速度显示，支持km/h和mph，适合横屏全屏显示",
     "author": "Yei.J. (AskaEth)",
     "version": "1.0.0",
+    "supported_games": "all",
     "config": {
         "icon": "🏎️",
         "unit": "kmh",
-        "show_unit": true,
         "aspect_ratio": "auto",
         "render_mode": "contain"
     }