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/bossPanel.tsx

@@ -239,7 +239,9 @@ const BossPanel = (): JSX.Element => {
   const [ challengingBossId, setChallengingBossId ] = useState<string | null>(
     null,
   );
-  const [ activeZoneId, setActiveZoneId ] = useState("verdant_vale");
+  const [ activeZoneId, setActiveZoneId ] = useState(() => {
+    return sessionStorage.getItem("elysium_boss_zone") ?? "verdant_vale";
+  });
   const [ showLocked, setShowLocked ] = useState(true);
 
   if (state === null) {
@@ -317,6 +319,11 @@ const BossPanel = (): JSX.Element => {
     }
   }
 
+  function handleZoneSelect(zoneId: string): void {
+    setActiveZoneId(zoneId);
+    sessionStorage.setItem("elysium_boss_zone", zoneId);
+  }
+
   function handleToggle(): void {
     setShowLocked((current) => {
       return !current;
@@ -374,7 +381,7 @@ const BossPanel = (): JSX.Element => {
 
       <ZoneSelector
         activeZoneId={activeZoneId}
-        onSelectZone={setActiveZoneId}
+        onSelectZone={handleZoneSelect}
         zones={zones}
       />