fix: PDF viewer - also wire up _render_file_viewer in dashboard.py view_page_root

The real route for /pages/{page_id} is in dashboard.py (no prefix router),
not board.py (prefix /board). Both routes now call _render_file_viewer
for content_format='file' pages.

app/routers/dashboard.py

@@ -235,6 +235,11 @@ async def view_page_root(request: Request, page_id: int):
             return HTMLResponse("<h2>Page not found</h2>", status_code=404)
     page = dict(row)
 
+    # For file-type pages (PDFs, images, etc.), use the dedicated file viewer
+    if page.get("content_format") == "file":
+        from app.routers.board import _render_file_viewer
+        return _render_file_viewer(page, request)
+
     ws = page.get("workspace", "")
     parts = ws.split("/") if "/" in ws else ["", ""]
     owner, repo = parts[0], parts[1] if len(parts) > 1 else ""