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/i18n/en.ts

@@ -12,6 +12,7 @@ export const en = {
   "common.remove": "Remove",
   "common.edit": "Edit",
   "common.close": "Close",
+  "common.clear": "Clear",
   "common.back": "Back",
   "common.next": "Next",
   "common.signOut": "Sign out",
@@ -78,6 +79,9 @@ export const en = {
   "mail.unpin": "Unpin",
   "mail.delete": "Delete",
   "mail.restore": "Restore",
+  "mail.emptyTrash": "Empty trash",
+  "mail.emptySpam": "Empty spam",
+  "mail.emptyConfirm": "Permanently delete everything in this folder? This can't be undone.",
   "mail.loadFailed": "Couldn't load this message.",
   "mail.imagesBlocked": "Images from this sender are blocked.",
   "mail.loadImages": "Load images",