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

backend/app/api/threads.py

@@ -83,12 +83,21 @@ async def list_threads(
     offset: int = Query(0, ge=0),
     filter: str = Query("all"),  # all | unread | pinned
     order: str = Query("desc"),  # desc (newest first) | asc (oldest first)
+    ids: str = Query(""),  # comma-separated thread ids (e.g. a plugin's label view)
 ) -> ThreadListOut:
     # Collect WHERE conditions once so the page query and the total-count query
     # stay in lockstep (offset pagination needs an accurate grand total).
     conditions: list[ColumnElement[bool]] = [Thread.user_id == user.id]
 
-    if view.startswith("account:"):
+    if ids:
+        # Explicit id set (ignores view/folder) — used by plugin-driven views like
+        # "all mail with label X", which supply the matching thread ids.
+        try:
+            wanted = [uuid.UUID(x) for x in ids.split(",") if x.strip()]
+        except ValueError:
+            raise_api_error(400, "bad_ids", "invalid thread id in ids")
+        conditions.append(Thread.id.in_(wanted or [uuid.uuid4()]))
+    elif view.startswith("account:"):
         # A single account's inbox: threads with a message in THIS account's
         # inbox mailbox. Keeps the per-account view consistent with the unified
         # Inbox (archived/trashed/sent threads don't leak in).