Labels in Mail UI (chips + Actions + sidebar), plain-text Lato, calendar-panel polish

Labels:
- Moved labelling out of the "+label" message slot into Mail: label chips on the
  thread subject bar (removable ✕) and before list subjects, and a Labels submenu
  in the per-message Actions menu (multi-select checkboxes + "Create new…").
  Per-message storage kept; the reading pane writes a thread-union aggregate so the
  list (no message ids) can show chips.
- Sidebar LABELS section rewritten: aligned monocolor tag rows, collapsible
  (collapsed by default), inline "+" create popover, hover "−" delete with a confirm
  popover, and click-to-filter the mail list by label.
- Backend: batch message/thread label reads, thread aggregate, create + delete
  (delete unhooks from all mail), and GET /labels/{name}/threads. Core GET /threads
  gained an `ids` param to power the label view. Removed the plugin-pages list from
  the Mail sidebar.

Plain-text mail now renders in Lato (self-hosted at /fonts, @font-face'd into the
sandboxed message iframe) instead of the monospace <pre> default.

Calendar right-panel (embed mode): restored the "…" view-options menu, pinned the
toolbar + mini-calendar (only the agenda scrolls), plain "All calendars" button
(inline resets), and the event popover now closes on any outside click.

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

frontend/src/i18n/en.ts

@@ -99,6 +99,10 @@ export const en = {
   "mail.emptySpam": "Empty spam",
   "mail.more": "More",
   "mail.markAllRead": "Mark all as read",
+  "mail.labels": "Labels",
+  "mail.noLabels": "No labels yet",
+  "mail.createLabel": "Create new…",
+  "mail.labelName": "Label name",
 
   "panel.toggle": "Toggle side panel",
   "panel.calendar": "Calendar",