embed mode: pass embed_mode to context, add conditional body class + CSS

- board.py: pass embed_mode=True to template context when ?embed=1
- base.html: <body class="embed-mode"> when embed_mode is true
- CSS: body.embed-mode hides .sidebar, .topbar, .unified-header,
  .header-actions; removes app-layout margin/padding; adjusts editor
  padding and max-width for peek panel
- Sidebar and header now properly hidden in iframe via CSS

app/routers/board.py

@@ -1005,7 +1005,8 @@ async def view_page(request: Request, page_id: int):
            "page_data": page_data,
            "breadcrumb_items": nav_crumbs,
            "nav_workspace_id": page.get("workspace_id") or 0,
-           "nav_page_id": page_id}
+           "nav_page_id": page_id,
+           "embed_mode": embed}
     template = env.get_template("page_editor_embed.html" if embed else "page_editor.html")
     response = template.render(**ctx)
     return HTMLResponse(content=response, headers={"Cache-Control": "no-store, max-age=0"})