fix(purse): don't show raw localization keys for unnamed seasonal currencies

A seasonal currency with no purse.seasonal.currency.<type> text rendered the raw key (e.g. 'purse.seasonal.currency.11') in the purse. Share PurseView's existing localizeWithFallback helper (extracted to api/utils with a pure resolveLocalized core + tests) and use it in SeasonalView with an empty fallback, so an unnamed currency shows just its icon + amount. Verified in-app.

src/api/utils/localizeWithFallback.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+
+import { resolveLocalized } from './localizeWithFallback';
+
+/**
+ * The localization manager returns the KEY itself when a translation is missing
+ * (LocalizationManager.getValue → `value || key`). `resolveLocalized` turns that
+ * "missing" signal into a caller-supplied fallback so raw keys like
+ * `purse.seasonal.currency.11` never reach the UI.
+ */
+describe('resolveLocalized', () =>
+{
+    it('returns the localized text when it differs from the key', () =>
+    {
+        expect(resolveLocalized('Pixels', 'purse.seasonal.currency.5', 'fallback')).toBe('Pixels');
+    });
+
+    it('returns the fallback when the text equals the key (missing translation)', () =>
+    {
+        expect(resolveLocalized('purse.seasonal.currency.11', 'purse.seasonal.currency.11', '')).toBe('');
+        expect(resolveLocalized('purse.seasonal.currency.11', 'purse.seasonal.currency.11', 'Currency')).toBe('Currency');
+    });
+
+    it('returns the fallback for empty / null text', () =>
+    {
+        expect(resolveLocalized('', 'some.key', 'FB')).toBe('FB');
+        expect(resolveLocalized(null as unknown as string, 'some.key', 'FB')).toBe('FB');
+    });
+});