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/components/thread/ThreadView.tsx

@@ -423,6 +423,8 @@ export function ThreadView({
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ["thread"] });
       queryClient.invalidateQueries({ queryKey: ["threads"] });
+      // Mark read/unread changes the Inbox unread badge.
+      queryClient.invalidateQueries({ queryKey: ["folder-counts"] });
     },
   });
   const move = useMutation({
@@ -431,6 +433,7 @@ export function ThreadView({
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ["thread"] });
       queryClient.invalidateQueries({ queryKey: ["threads"] });
+      queryClient.invalidateQueries({ queryKey: ["folder-counts"] });
     },
   });
 
@@ -518,7 +521,7 @@ export function ThreadView({
             className="rounded p-1.5 text-app-muted hover:bg-app-hover hover:text-app-fg"
             title={t("mail.print")}
             aria-label={t("mail.print")}
-            onClick={() => void printMessages(subject, messages)}
+            onClick={() => void printMessages(subject, visual)}
           >
             <Icon name="print" size={16} />
           </button>