Viewer: search filtering, pagination, and per-sender hide with show-hidden toggle

[zbuc]

Closes #21

Summary

Front-end only (web/ — the demo inherits these once #20 merges; no server or Rust changes):

• Search: case-insensitive substring filter over sender addresses; resets to page 1 on input.
• Pagination: client-side with a page-size selector (25/50/100/500, default 50, persisted), prev/next with "Page X of Y · N senders" info; page clamps when the row set shrinks.
• Hide (🙈) per row: removes the sender from the list, subtracts its count from Total messages, and recomputes the distinct-senders and top-sender tiles. Persisted in localStorage, so hides survive reload.
• Show hidden toggle: reveals hidden rows dimmed with an unhide (👁) button; totals continue to exclude hidden senders while shown. Page info notes "(N hidden)" when hides are active.

Sender strings remain rendered exclusively via textContent (attacker-controlled From: headers).

Testing

• End-to-end against the real web binary with a synthetic 130-sender stats.db: page renders under the server's CSP with 50 rows and correct pager ("Page 1 of 3 · 130 senders").
• Scripted interaction test in headless Chrome against the 84k demo dataset: search "chatterloop" narrows 1,419 → 8 senders; hiding the top sender subtracts exactly its 6,842 count (84,000 → 77,158) and promotes the next sender in the top tile; show-hidden reveals the dimmed row; localStorage holds the hidden list; unhide restores 84,000; next/prev paging works with correct disabled states.
• Verified Generate the Pages demo from web/ at deploy time #20's build_demo.py still transforms the new assets cleanly (all anchors intact), so the Pages demo build keeps working whichever merges first.