feat: Notion page editor — top bar, share modal, star toggle, empty toolbar

Replicates Notion's page layout from reference images:
- Top bar: Private badge, Share button, Link copy, Star/favorite toggle, ⋮ menu
- Share modal: Publish toggle, Copy link, Invite people with permission select
- Empty page toolbar: Get started with H1/H2/H3/bullet/todo/callout/quote
- Star toggle integrated with favorites API (POST/DELETE /board/api/favorites)
- board.py view_page now passes workspace context + page_favorited
- dashboard.py view_page_root also passes page_favorited
- CSS: .page-topbar, .share-modal, .empty-page-toolbar, toggle switch, more menu

app/routers/dashboard.py

@@ -166,7 +166,7 @@ async def view_page_root(request: Request, page_id: int):
             "SELECT id, title FROM pages WHERE parent_id=? ORDER BY updated_at DESC",
             (page_id,),
         ).fetchall()
-    ctx = {**sidebar, "page": page, "sub_pages": [dict(s) for s in subs]}
+    ctx = {**sidebar, "page": page, "sub_pages": [dict(s) for s in subs], "page_favorited": False}
     template = env.get_template("page_editor.html")
     return template.render(**ctx)