fix: correct TemplateResponse call in mail_sources_page to use new Starlette API

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/16838094-12d6-4279-9c94-58b7fb4ad3d7

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/main.py

@@ -345,7 +345,7 @@ async def settings_page(request: Request):
 
 @app.get("/mail-sources", response_class=HTMLResponse)
 async def mail_sources_page(request: Request):
-    return templates.TemplateResponse("mail_sources.html", {"request": request})
+    return templates.TemplateResponse(request, "mail_sources.html")
 
 
 @app.get("/upload", response_class=HTMLResponse)