feat(static): add frontend token auto-refresh for dnfs page

- New token_manager.js: silent refresh timer (80% lifetime) + global
  fetch 401 interceptor with one-shot retry.
- dnfs.py /whoami now returns iat, created_at, keep_time for expiry
  estimation.
- Dnfs.html loads token_manager.js before dnfs.js (zero change to dnfs.js).
- Graceful fallback: stale tokens without new fields, refresh failures,
  and /refresh_token 401 itself all degrade to the existing 401→login flow.

static/public/dnfs.html

@@ -26,6 +26,7 @@
 			<!--<script src="https://cdnjs.cloudflare.com/ajax/libs/markdown-it/12.0.0/markdown-it.min.js"></script>-->
 			<script src="js/markdown-it-12_0_0.js"></script>
 			<script src="js/validation.js"></script>
+			<script src="js/token_manager.js"></script>
 			<script src="js/dnfs.js"></script>
 		</div>
 		<footer class="footer">