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

frontend/src/lib/ws.ts

@@ -47,16 +47,22 @@ export function connectWs(queryClient: QueryClient): () => void {
       queryClient.invalidateQueries({ queryKey: ["threads"] });
       queryClient.invalidateQueries({ queryKey: ["quota"] });
       queryClient.invalidateQueries({ queryKey: ["drafts-remote"] });
+      queryClient.invalidateQueries({ queryKey: ["folder-counts"] });
       if (typeof payload.thread_id === "string") {
         queryClient.invalidateQueries({ queryKey: ["thread", payload.thread_id] });
       }
     }
+    if (name === "flags.changed") {
+      // Read/unread changes affect the Inbox unread badge.
+      queryClient.invalidateQueries({ queryKey: ["folder-counts"] });
+    }
     if (name === "account.state_changed") {
       queryClient.invalidateQueries({ queryKey: ["accounts"] });
     }
     if (name === "message.sent") {
       // The sent message's draft is deleted server-side; refresh the Drafts list.
       queryClient.invalidateQueries({ queryKey: ["drafts"] });
+      queryClient.invalidateQueries({ queryKey: ["folder-counts"] });
     }
   };