UI: unified design system, light/dark themes, redesigned pages and player UI

Tokens and components in styles.css, shared Ui (dialogs, toasts), icons,
bottom tab bar on mobile, keyboard/gamepad navigation, library shelf with
filters and conversion badges, hero game page, restyled player screens.
Games list now returns per-game version counts. Adds docs/UI_DESIGN.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/api.ts

@@ -41,6 +41,9 @@ export interface Game {
   engine: string;
   cover_hash: string | null;
   created_at: string;
+  ready_versions?: number;
+  pending_versions?: number;
+  failed_versions?: number;
 }
 
 export interface GameDetail extends Game {