feat: local-first feed loading

[strdr4605]

Closes #93

Summary

• Show cached feed items immediately on load using storedFeed.items as placeholderData
• Persist fresh items back to IndexedDB after each successful RSS fetch
• Add 3 E2E tests covering cache-on-load, DOM update, and DB persistence

Test Plan

• On reload with blocked network, cached feed title and articles are visible immediately
• After network fetch with updated feed, new articles appear in DOM
• After network fetch, reload with blocked network still shows updated articles from DB