fix: persist UI preferences across navigation and sessions (#48)

## Summary

- **#35** — Adventure multiplier selection is now persisted in `localStorage` (`"elysium_batch_size"`). The chosen batch size is restored automatically on the next visit, with a graceful fallback to `1` for missing or unrecognisable values.
- **#36** — Zone selection in the boss panel and quest panel is now persisted in `sessionStorage` (`"elysium_boss_zone"` / `"elysium_quest_zone"`). The selected zone survives navigation within a session and resets cleanly when the session ends, defaulting to Verdant Vale if no stored value exists.

## Test plan

- [x] Lint — zero errors, zero warnings
- [x] Build — all packages build cleanly
- [x] Tests — 415 tests passing, 100% coverage across all packages
- [ ] Manual: select a non-default batch size, refresh the page — multiplier should be restored
- [ ] Manual: switch to a non-default zone in the boss panel, navigate away and back — zone should still be selected
- [ ] Manual: repeat for the quest panel
- [ ] Manual: log out and back in — zone selection should reset to Verdant Vale

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #48
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

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

@@ -67,7 +67,9 @@ interface CollectResult {
 const ExplorationPanel = (): JSX.Element => {
   const { state, startExploration, collectExploration, formatNumber }
     = useGame();
-  const [ activeZoneId, setActiveZoneId ] = useState("verdant_vale");
+  const [ activeZoneId, setActiveZoneId ] = useState(() => {
+    return sessionStorage.getItem("elysium_explore_zone") ?? "verdant_vale";
+  });
   const [ pendingAreaId, setPendingAreaId ] = useState<string | null>(null);
   const [ lastResult, setLastResult ] = useState<CollectResult | null>(null);
 
@@ -116,6 +118,7 @@ const ExplorationPanel = (): JSX.Element => {
   function handleZoneSelect(id: string): void {
     setActiveZoneId(id);
     setLastResult(null);
+    sessionStorage.setItem("elysium_explore_zone", id);
   }
 
   const goldChange = lastResult?.response.event?.goldChange ?? 0;