fix: resolve all 8 open bug tickets (#242–#249) (#250)

## Summary

- **#242** — Crystals in the resource bar now use `formatNumber` to respect the player's notation setting (suffix/scientific/engineering)
- **#243** — Companion unlock progress includes current-run gold (`totalGoldEarned`) on both client and server, so companions unlock at the correct threshold
- **#244** — Empty green reward bubbles no longer render for quest crystal rewards with a zero amount
- **#245/#248** — Auto-save skips when `isAutoPrestigingReference.current` is true, preventing it from racing with an in-flight prestige and breaking the optimistic lock
- **#246** — Generated and uploaded CDN images for `crystal_pulse`, `crystal_surge`, and `crystal_tempest` upgrades
- **#247** — `validateAndSanitize` merges daily challenge progress by taking the max of client vs. server progress per challenge, so stale auto-saves can no longer roll back server-side completions
- **#249** — Cached save signature is cleared after `buyPrestigeUpgrade` succeeds, preventing a stale-signature mismatch on the next auto-save

## Test plan

- [ ] Lint passes (`pnpm lint`)
- [ ] Build passes (`pnpm build`)
- [ ] Tests pass with 100% coverage (`pnpm test`)
- [ ] Crystals display in resource bar respects notation setting
- [ ] No empty reward bubbles on quests that don't award crystals
- [ ] Companion progress bar shows correct value including current-run gold
- [ ] Auto-prestige no longer causes save errors
- [ ] Crafting a recipe updates daily challenge progress persistently (not rolled back by next auto-save)
- [ ] Buying a prestige upgrade does not cause a signature mismatch error on next save
- [ ] Crystal upgrade images display correctly in-game

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #250
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/context/gameContext.tsx

@@ -1356,10 +1356,11 @@ export const GameProvider = ({
         newlyFailedQuestsReference.current = [];
       }
 
-      // Auto-save every 30 seconds (skip if a force sync is in-flight to avoid signature collisions)
+      // Auto-save every 30 seconds (skip if a force sync or auto-prestige is in-flight to avoid signature collisions)
       if (Date.now() - lastSaveReference.current >= autoSaveIntervalMs) {
         lastSaveReference.current = Date.now();
-        if (stateReference.current !== null && !isSyncingReference.current) {
+        // eslint-disable-next-line stylistic/max-len -- Long condition; splitting would reduce readability
+        if (stateReference.current !== null && !isSyncingReference.current && !isAutoPrestigingReference.current) {
           void saveGame({
             state: stateReference.current,
             ...signatureReference.current === null
@@ -1856,6 +1857,13 @@ export const GameProvider = ({
           },
         };
       });
+
+      /*
+       * Buying a prestige upgrade mutates DB state; clear the cached signature
+       * so the next auto-save doesn't collide with a stale one.
+       */
+      signatureReference.current = null;
+      localStorage.removeItem("elysium_save_signature");
     } catch (error_: unknown) {
       logError("buy_prestige_upgrade", error_);
       // Silently ignore — server errors shouldn't crash the UI