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/adventurerPanel.tsx

@@ -16,6 +16,31 @@ import type { Adventurer } from "@elysium/types";
 type BatchSize = 1 | 5 | 10 | 25 | 100 | "max";
 const batchOptions: Array<BatchSize> = [ 1, 5, 10, 25, 100, "max" ];
 
+/**
+ * Parses a localStorage string back into a valid BatchSize, defaulting to 1.
+ * @param stored - The raw string from localStorage (or null if absent).
+ * @returns A valid BatchSize value.
+ */
+const parseBatchSize = (stored: string | null): BatchSize => {
+  if (stored === "max") {
+    return "max";
+  }
+  const numeric = Number(stored);
+  if (numeric === 5) {
+    return 5;
+  }
+  if (numeric === 10) {
+    return 10;
+  }
+  if (numeric === 25) {
+    return 25;
+  }
+  if (numeric === 100) {
+    return 100;
+  }
+  return 1;
+};
+
 /**
  * Computes the total cost to buy a batch of adventurers.
  * @param adventurer - The adventurer to buy.
@@ -148,7 +173,9 @@ const AdventurerCard = ({
 const AdventurerPanel = (): JSX.Element => {
   const { state, formatNumber } = useGame();
   const [ showLocked, setShowLocked ] = useState(true);
-  const [ batchSize, setBatchSize ] = useState<BatchSize>(1);
+  const [ batchSize, setBatchSize ] = useState<BatchSize>(() => {
+    return parseBatchSize(localStorage.getItem("elysium_batch_size"));
+  });
 
   if (state === null) {
     return (
@@ -196,6 +223,7 @@ const AdventurerPanel = (): JSX.Element => {
         {batchOptions.map((option) => {
           function handleBatchSelect(): void {
             setBatchSize(option);
+            localStorage.setItem("elysium_batch_size", String(option));
           }
           return (
             <button