Implement the 008-persist-encounter feature that saves encounter state to localStorage so it survives page reloads

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/008-persist-encounter/quickstart.md

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+# Quickstart: Persist Encounter
+
+## What This Feature Does
+
+Saves the current encounter to browser localStorage so it survives page reloads. No user action required -- saving and restoring happens automatically.
+
+## Key Files
+
+| File | Purpose |
+|------|---------|
+| `apps/web/src/persistence/encounter-storage.ts` | New: read/write/validate encounter in localStorage |
+| `apps/web/src/hooks/use-encounter.ts` | Modified: load from storage on init, persist on change |
+
+## How It Works
+
+1. **On app load**: The `useEncounter` hook calls `loadEncounter()` from the storage module. If valid data exists, it initializes state from it. Otherwise, the demo encounter is used.
+2. **On every state change**: A `useEffect` watches the encounter state and calls `saveEncounter()` to write it to localStorage.
+3. **On error**: All storage operations are wrapped in try/catch. Failures are silent -- the app continues working in memory-only mode.
+
+## Testing
+
+```bash
+pnpm vitest run apps/web/src/persistence/__tests__/encounter-storage.test.ts
+```
+
+Tests cover:
+- Round-trip serialization (save then load returns same encounter)
+- Invalid/corrupt data returns null
+- Missing fields return null
+- Non-JSON data returns null
+
+## Manual Verification
+
+1. Run `pnpm --filter web dev`
+2. Add/remove combatants, set initiative, advance turns
+3. Refresh the page -- encounter state should be preserved
+4. Open DevTools > Application > localStorage to inspect the stored data
+5. Delete the storage key and refresh -- demo encounter should appear
+6. Set the storage value to `"garbage"` and refresh -- demo encounter should appear