fix: PDF viewer - wire up _render_file_viewer + URL-encode filenames

- view_page now calls _render_file_viewer for content_format='file' pages
  instead of rendering the full page_editor template (which was the old path)
- URL-encode filenames in file_url using urllib.parse.quote() to handle
  spaces and special characters correctly
- Fixed in both board.py (viewer + page_data) and dashboard.py

The _render_file_viewer was dead code — defined but never called. File pages
were going through page_editor.html which rendered PDFs in an iframe embedded
in the editor UI. Now they get a clean standalone HTML viewer.

app/routers/dashboard.py

@@ -270,7 +270,8 @@ async def view_page_root(request: Request, page_id: int):
                 ws_id = p.replace("workspace_", "")
                 break
         filename = fp_parts[-1] if fp_parts else page.get("title", "File")
-        file_url = f"/api/files/{ws_id}/{filename}" if ws_id else ""
+        from urllib.parse import quote
+        file_url = f"/api/files/{ws_id}/{quote(filename, safe='')}" if ws_id else ""
         page_data["file_url"] = file_url
         page_data["file_mime"] = mime_type
         page_data["file_size"] = file_size