peek panels: reusable editor module — shared _page_editor_content.html + embed template

Architecture for zero code duplication:
- Extract page editor HTML into _page_editor_content.html (shared include)
- page_editor.html includes it (same behavior as before)
- page_editor_embed.html includes it with empty topbar (no header in peek)
- board.py: ?embed=1 renders page_editor_embed.html

Peek panels now load /pages/{id}?embed=1 in iframe:
- Editor renders without sidebar or header breadcrumb/actions
- Full editing capability (blocks, markdown, slash commands, save)
- Same code — one change to _page_editor_content.html updates everywhere

app/routers/board.py

@@ -946,7 +946,9 @@ async def delete_page(request: Request, page_id: int):
 
 @router.get("/pages/{page_id}", response_class=HTMLResponse)
 async def view_page(request: Request, page_id: int):
-    """Render a page as HTML, or a file viewer for uploaded files."""
+    """Render a page as HTML, or a file viewer for uploaded files.
+    ?embed=1 — minimal mode for side peek (editor only, no header)."""
+    embed = request.query_params.get("embed") == "1"
     from jinja2 import Environment, FileSystemLoader
     env = Environment(loader=FileSystemLoader("app/templates"))
     with get_conn() as conn:
@@ -1004,7 +1006,7 @@ async def view_page(request: Request, page_id: int):
            "breadcrumb_items": nav_crumbs,
            "nav_workspace_id": page.get("workspace_id") or 0,
            "nav_page_id": page_id}
-    template = env.get_template("page_editor.html")
+    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"})