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/messages.py

@@ -37,6 +37,14 @@ _INLINE_SAFE = {"image/png", "image/jpeg", "image/gif", "image/webp", "applicati
 _DOC_TEMPLATE = """<!doctype html><html><head><meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <style>
+  /* Self-hosted Lato so plain-text bodies use the app font (the sandboxed iframe
+     can't inherit the parent's web fonts). Served from the frontend's /fonts. */
+  @font-face{{font-family:"Lato";src:url("/fonts/Lato-Regular.ttf") format("truetype");
+    font-weight:400;font-style:normal;font-display:swap}}
+  @font-face{{font-family:"Lato";src:url("/fonts/Lato-Bold.ttf") format("truetype");
+    font-weight:700;font-style:normal;font-display:swap}}
+  @font-face{{font-family:"Lato";src:url("/fonts/Lato-Italic.ttf") format("truetype");
+    font-weight:400;font-style:italic;font-display:swap}}
   html,body{{margin:0;padding:0}}
   body{{font-family:system-ui,-apple-system,"Segoe UI",Roboto,sans-serif;font-size:14px;
     line-height:1.5;color:#1a1a1a;background:#fff;padding:14px;word-break:break-word}}
@@ -45,6 +53,10 @@ _DOC_TEMPLATE = """<!doctype html><html><head><meta charset="utf-8">
   blockquote{{margin:0 0 0 12px;padding-left:12px;border-left:3px solid #e0e0e0;color:#555}}
   a{{color:#2563eb}}
   pre{{white-space:pre-wrap;word-wrap:break-word}}
+  /* Plain-text emails: proportional Lato instead of the monospace <pre> default. */
+  .wm-plaintext{{white-space:pre-wrap;word-wrap:break-word;
+    font-family:"Lato",system-ui,-apple-system,"Segoe UI",Roboto,sans-serif;
+    font-size:15px;line-height:1.55}}
 </style></head><body>{body}</body></html>"""
 
 
@@ -112,7 +124,7 @@ async def message_html(
         return HTMLResponse(content=_wrap(html_part.content))
     text_part = next((p for p in parts if p.kind == "text"), None)
     body = html_lib.escape(text_part.content) if text_part else ""
-    return HTMLResponse(content=_wrap(f"<pre>{body}</pre>"))
+    return HTMLResponse(content=_wrap(f'<div class="wm-plaintext">{body}</div>'))
 
 
 @router.post("/{message_id}/allow-remote-images", dependencies=[Depends(require_csrf)])