History list — show recently saved articles in popup

Add a tab to popup showing the most recent 20 saved articles: slug + time + commit URL. Lets users quickly revisit what they captured.

Implementation sketch:

• New src/lib/storage/history.ts with chrome.storage.local ring buffer (20 entries)
• Push { slug, title, url, commitSha, savedAt } after each successful save
• Popup gains a tab toggle, new pane renders the list