feat: reusable, time-windowed leaderboard (@cozy-games/leaderboard)

Extract the leaderboard into a generic, backend-agnostic package and add
rolling Today/Week/Month/All-Time windows, a web component, and local-first
development against the Firestore emulator.

Package (leaderboard/):
- LeaderBoardService: backend-agnostic core via a storage-adapter seam, with
  Firebase and Supabase adapters (Supabase client injected, no added dep)
- Rolling time windows (last 24h / 7d / 30d) with hover tooltips; top-N by score
- <cozy-leaderboard> web component built on web-component-base: compose the UI
  in HTML, configure the backend once in JS
- Every user-facing string is configurable (labels, tooltips, empty/loading/
  error messages, anonymous name) so i18n lives in the app; README + CONFIGURATION

App (mnswpr.com):
- Compose the board declaratively in index.html via <cozy-leaderboard>
- Nickname + randomized greeting bar; score submission through the element
- Legends: the current all-time leaders frozen into a static /legends page
- Firebase config and leaderboard namespace via Vite env vars; emulator-first
  local dev (VITE_FIRESTORE_EMULATOR)

Firebase schema-as-code:
- firebase.json, .firebaserc, firestore.rules (public reads, create-only scores,
  no client updates/deletes, namespace-generalized), empty indexes (rolling
  windows need none)
- prod (mw-*) vs dev/test (mw-test-*) separation by collection namespace
- emulator config, seed script, and docs (firebase-leaderboards.md,
  leaderboard-env-migration.md, AYO.md)

Utils/tests: UTC date-bucket helper (retained as metadata) with Vitest coverage.

README.md

@@ -1,7 +1,3 @@
-> [!IMPORTANT]
-> **mnswpr has a new home.** Development now happens in the **Cozy Games** monorepo:
-> **[git.ayo.run/ayo/cozy-games](https://git.ayo.run/ayo/cozy-games)**.
-
 # Play Minesweeper Online for Free
 [![Netlify Status](https://api.netlify.com/api/v1/badges/172478bd-afc5-4e47-95ba-d9ab814248fb/deploy-status)](https://app.netlify.com/sites/mnswpr/deploys)