test: add cache-hit path tests for client/modules/search.ts

[felladrin]

Split off from #2130.

We test performSearch, but nothing asserts that a repeated query is served from the IndexedDB cache and skips the network fetch — that's the module's headline feature and it's currently uncovered.

What changed

• client/setupTests.ts — Wired fake-indexeddb/auto so Dexie-based cache tests exercise the real IndexedDB code paths instead of silently erroring into catch blocks.
• package.json — Added fake-indexeddb to devDependencies.
• client/modules/search.test.ts — New Cache Hit Path describe block with 7 tests.

Tests added

Test	What it proves
Repeated text query served from cache	Second searchText call skips fetch; log entry and textHits counter confirm the cache-hit branch.
Repeated image query served from cache	Same for searchImages with imageHits counter.
Different queries → separate entries	Two distinct queries each hit the network.
Different limits → separate entries	Same query, different limit values produce separate cache keys.
TTL expiry forces refetch	updateCacheConfig({ ttl: 0 }) between calls triggers a second fetch.
Caching disabled skips cache	enabled: false makes both calls hit the network.
Text/image stores independent	Same query through searchText and searchImages produces 2 fetches; a third call to the image store hits the cache.

Verification

• 364 tests pass across 40 files.
• Lint clean (Biome, TypeScript, knip, jscpd, architectural linter).
• Reviewed by Claude (3 rounds) — approved with no blockers.