Add user management with admin and viewer roles

- data/users.json store; existing single account migrates to first admin
- Roles: admin (settings, import, users) and viewer (read-only, own password)
- Live role/disabled lookup; per-user session epoch so password changes,
  resets and disabling sign that user out everywhere
- Login brute-force lockout (5 fails -> 5 min), constant-time unknown-user check
- /api/users CRUD with last-admin and self-lockout safeguards
- Account page (own password + admin user management); role-aware header
- Min password length 8; recovery token resets the primary admin and clears lockouts

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/public/dashboard-ui.js

@@ -549,6 +549,24 @@ function renderInsights() {
   });
 }
 
+// ── Who's signed in: role-aware header links ─────────────────────────────────
+(async function roleAwareHeader() {
+  try {
+    const me = await getJSON('/api/dashboard/me');
+    const right = document.querySelector('.header-right');
+    if (!right) return;
+    const settings = right.querySelector('a[href="/dashboard/settings"]');
+    if (me.role !== 'admin' && settings) settings.remove();
+    const acct = document.createElement('a');
+    acct.href = '/dashboard/account'; acct.className = 'logout'; acct.textContent = me.role === 'admin' ? 'Account & Users' : 'Account';
+    acct.title = `Signed in as ${me.username} (${me.role})`;
+    (settings && me.role === 'admin' ? settings : right.querySelector('a[href="/dashboard/logout"]'))?.before(acct);
+    const nm = $('stationName');
+    if (nm && !nm.textContent) nm.textContent = me.username;
+    window.DASH_USER = me;
+  } catch {}
+})();
+
 // ── Data refresh ─────────────────────────────────────────────────────────────
 async function refreshData() {
   try {