feat: extensible leaderboard with local provider and async provider interface

[Copilot]

Implements a local personal leaderboard designed to support a future global backend (e.g. Supabase) without refactoring call sites.

Architecture

• LeaderboardProvider interface — getEntries() / addEntry() / isHighScore(), all async. New backends implement this and nothing else changes.
• LocalLeaderboardProvider — localStorage, Promise-wrapped, keeps top 10 entries sorted by score.
• LeaderboardManager — accepts providers: LeaderboardProvider[], fans writes to all providers, merges and deduplicates reads by entry.id.

Adding Supabase later is a one-line change in game.ts:

new LeaderboardManager([new LocalLeaderboardProvider(), new SupabaseLeaderboardProvider()])

LeaderboardEntry shape

interface LeaderboardEntry {
    id: string;            // crypto.randomUUID() — deduplicates across providers
    name: string;
    score: number;
    timeRemaining: number;
    timestamp: number;
    source: "local" | "global";  // lets UI distinguish provider origin
}

Game flow

• endGame() triggers a leaderboard overlay (name-entry form, pre-filled with last used name).
• On submit: saves entry → fetches merged entries → shows ranked table with the new row highlighted.
• SPACE is gated in startGame(): no-op during name-entry; dismisses the table and starts a new game when the table is visible.

UI

New overlay panel with two phases (name-entry / score table), synthwave-styled to match the existing design system. Loading state is built in from the start so the async path works transparently when a remote provider is added.