feat: persist watchlist and bridge filter/sort preferences across sessions

[Mona-i]

Closes #12

Summary

Implements full localStorage persistence for the watchlist and bridge filter/sort state so returning users no longer need to re-apply preferences on every visit.

Changes

src/stores/watchlistStore.ts (new)

• Zustand persist store for the watchlist, namespaced key swipely:watchlist (v1)
• normalizeWatchlistState validates/sanitizes persisted shapes; corrupt data falls back to defaults
• migrate callback handles version upgrades gracefully
• merge callback migrates the legacy swipely.watchlists.v1 key on first load, then removes it
• clearActiveWatchlist removes the localStorage key when all lists are empty

src/stores/bridgeFilterSortStore.ts (new)

• Zustand persist store for bridge statusFilter + sortBy, namespaced key swipely:bridge-filter-sort (v1)
• normalizeBridgeFilterSortState rejects unknown values and falls back to safe defaults
• Includes applyBridgeFilterSort helper and getBridgeHealthScore utility

src/hooks/useWatchlist.ts

• Replaced React-context + raw localStorage implementation with thin facade over useWatchlistStore
• WatchlistProvider kept as no-op pass-through for call-site compatibility

src/components/BridgeFilterSort.tsx

• Now reads from / writes to useBridgeFilterSortStore when not controlled by parent props
• Fully backwards-compatible: controlled prop usage unchanged

src/pages/Bridges.tsx

• Wires applyBridgeFilterSort into the filteredBridges memo so persisted filter/sort is applied on every render
• Mounts BridgeFilterSort in the toolbar alongside favorites and refresh controls

src/stores/index.ts

• Exports both new stores and their helpers

Tests

• src/stores/watchlistStore.test.ts: rehydration, legacy migration, corrupt-data resilience, clearActiveWatchlist removes persisted state
• src/stores/bridgeFilterSortStore.test.ts: rehydration, invalid-value guards, corrupt-data resilience, filter/sort utility functions
• src/hooks/useWatchlist.test.tsx: updated storage-key assertion to the new namespaced key

Acceptance Criteria

• Watchlist survives reload and browser restart: Zustand persist writes to swipely:watchlist on every mutation
• Bridge filter/sort selections are restored on return: Zustand persist writes to swipely:bridge-filter-sort on every change
• Corrupt/outdated persisted data is handled without crashing: normalizeWatchlistState / normalizeBridgeFilterSortState + migrate callbacks
• Clearing the watchlist clears persisted state: clearActiveWatchlist removes localStorage key when all lists are empty

CI Validation

• npm run lint: 0 errors
• npm run type-check: 0 errors
• npm run test: 56 test files, 603 tests all passing
• npm run build: production build succeeds