mail list redesign, folder actions, empty-folder, unread badges & empty-state art

Frontend:
- Redesign the mail list rows (avatar + sender + subject/snippet + right rail),
  even-spaced attachment/pin/archive/trash icons, hover-reveal actions, pink
  always-on pin, monocolor paperclip, row gaps, wider sender-subject gap.
- Sidebar folder badges: Inbox unread + Drafts counts (live via WS + mutations).
- Trash/Spam/Archive: hover Restore icon (replaces Archive), permanent delete in
  Trash; "Empty trash" / "Empty spam" buttons on the sort bar.
- Reading pane empty state: email illustration above an enlarged
  "Select a conversation".
- Search field clear (X) button; mark-read after a 4s dwell instead of instantly.

Backend:
- GET /api/mailboxes/counts (inbox unread + drafts) for the sidebar badges.
- POST /api/folders/{special}/empty + empty_folder_job to permanently clear a
  Trash/Spam/Archive folder (server EXPUNGE + DB rows + thread cleanup).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/tests/api/test_api.py

@@ -298,6 +298,22 @@ async def test_thread_detail_hides_trashed_message_unless_viewing_trash(api: Api
     assert trow["message_count"] == 1
 
 
+async def test_folder_counts(api: ApiCtx) -> None:
+    assert api.session is not None
+    # All three seeded inbox messages are unread; no drafts yet.
+    resp = await api.client.get("/api/mailboxes/counts")
+    assert resp.status_code == 200
+    assert resp.json() == {"inbox_unread": 3, "drafts": 0}
+
+    # Marking one message \Seen lowers the unread count.
+    msg = await api.session.get(Message, api.message_ids[0])
+    assert msg is not None
+    msg.flags = ["\\Seen"]
+    await api.session.commit()
+    body = (await api.client.get("/api/mailboxes/counts")).json()
+    assert body["inbox_unread"] == 2
+
+
 async def test_message_html_is_sanitized(api: ApiCtx) -> None:
     # the lunch message has HTML with a script
     resp = await api.client.get("/api/threads")