Add refresh countdown, fix quota fs lookup, update README

- main.js: show live countdown "Updated HH:MM:SS — next in Xs" so the
  30s auto-refresh is visually obvious instead of just a static timestamp
- app.py: rename _find_mountpoint → _find_quota_fs and fix the stale
  call site that caused a NameError; function now returns mountpoint from
  df, falling back to the path itself if df returns /
- README: add full quota setup section (fstab usrquota, quotacheck,
  quotaon, verify with repquota); fix stale "127.0.0.1" bind note
  (service already uses 0.0.0.0); update Auth section (sessions, not
  Basic Auth)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/js/dashboard.js

@@ -57,5 +57,5 @@ async function loadDashboard() {
         }).join('');
     }
 
-    document.getElementById('lastRefresh').textContent = new Date().toLocaleTimeString();
+    _startRefreshCountdown();
 }