Add a local favorites watchlist

## Why

Let people keep a small personal watchlist while browsing the demo. This is intentionally local to one browser: no TMDB account, authentication, or write API is involved.

## Scope

- Store the minimum movie summary needed for a watchlist in `localStorage` under one versioned key.
- Add an accessible favorite/unfavorite control to each discovery card and the movie detail view.
- Add `/favorites` and link it from the primary navigation.
- The favorites view handles both populated and empty states and links back to each movie detail page.
- Hydration and unavailable/corrupt storage must fail safely without breaking browsing.

## Acceptance criteria

- Favoriting from a discovery card updates that control and makes the movie appear at `/favorites` without a reload.
- The same saved state is reflected on the matching detail page; removing it from either surface removes it from `/favorites`.
- Refreshing the page preserves the watchlist in the same browser.
- With no saved items, `/favorites` shows an intentional empty state with a route back to Discover.
- Controls expose an accurate accessible name and pressed state and work by keyboard.
- A new pure unit test exercises add, remove, deduplication, corrupt-data fallback, and storage round-tripping. No existing test file is changed.
- `npm run typecheck`, `npm run lint`, `npm test`, and `TMDB_USE_MOCKS=true npm run build` pass.
- TMDB attribution remains visible and `TMDB_API_KEY` remains server-only.

## Guardrails

Keep this under 400 changed lines and within at most eight files. Do not touch `.env*`, `lib/tmdb.ts`, `next.config.ts`, package files, GitHub workflows, or factory policy/adapters. Do not add a dependency.

## Expected files

`lib/favorites.ts`, `lib/favorites.test.ts` (new), `components/favorite-button.tsx`, `components/movie-card.tsx`, `app/movies/[id]/page.tsx`, `app/favorites/page.tsx`, `app/layout.tsx`, `app/globals.css`