feat: communicate exploration zone unlock conditions in-game (#74)

## Summary

- Locked exploration zones now show a `🔒 This zone is locked. Unlock exploration by:` hint above the area list, with the specific `⚔️ Defeat: {boss}` and `📜 Complete: {quest}` required
- Updated the About panel's Exploration how-to-play entry to document the zone unlock rule explicitly
- No new data required — unlock conditions are read directly from `zone.unlockBossId` and `zone.unlockQuestId` already in state

## Test plan

- [ ] Verify locked exploration zones display the correct boss and quest unlock hints
- [ ] Verify already-unlocked zones show no hint
- [ ] Verify starter zone (no unlock conditions) shows no hint
- [ ] Verify the About panel Exploration entry reflects the updated description
- [ ] Confirm lint, build, and tests all pass

Closes #59

Reviewed-on: #74
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/components/game/aboutPanel.tsx

@@ -111,7 +111,11 @@ const howToPlay = [
       + " real-time and reward gold, essence, and crafting materials when"
       + " collected. Each area has a set duration — short explorations are"
       + " faster but longer ones offer rarer finds. A 📖 icon marks areas"
-      + " you've collected from at least once, unlocking a Codex entry.",
+      + " you've collected from at least once, unlocking a Codex entry."
+      + " Exploration zones are locked until the corresponding main-game"
+      + " zone is unlocked — which requires defeating that zone's final boss"
+      + " and completing its final quest. The Exploration tab shows the"
+      + " specific boss and quest required for each locked zone.",
     title: "🗺️ Exploration",
   },
   {