feat(settings): keep backup pwd on disable, dedicated pwd button, tab icons

Auto-backup no longer wipes the stored password when disabled, so
re-enabling reuses it silently. A dedicated "Set/Change backup password"
button (mirrors sync) owns the password, with a warning status when
unset. Corrected the stale hint that claimed the backup pwd was derived
from the master password. Added icons to each settings tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.backup.js

@@ -59,6 +59,18 @@ function refreshAutoBackupUI(cfg) {
     $('#autoBackupLast').textContent = cfg.last
         ? 'Last run: ' + cfg.last.replace('T', ' ').slice(0, 16)
         : 'Never run yet';
+    $('#autoBackupSetPwdBtn').textContent = cfg.hasPwd
+        ? 'Change backup password' : 'Set backup password';
+    const pwdStatus = $('#autoBackupPwdStatus');
+    if (cfg.hasPwd) {
+        pwdStatus.innerHTML = '';
+        pwdStatus.style.color = 'var(--text-faint)';
+        pwdStatus.textContent = 'Set.';
+    } else {
+        pwdStatus.style.color = 'var(--warning, #e0a800)';
+        pwdStatus.innerHTML = '<svg width="13" height="13"><use href="#i-alert"/></svg>' +
+            '<span>Not set — backups won\'t run</span>';
+    }
 }
 
 async function pickAutoBackupFolder() {
@@ -100,27 +112,32 @@ async function promptAndStoreBackupPwd() {
 async function onToggleAutoBackup(ev) {
     const enabled = ev.target.checked;
     if (enabled) {
-        const cfg = await loadAutoBackupConfig();
-        if (!cfg.hasPwd && !(await promptAndStoreBackupPwd())) {
-            ev.target.checked = false;
-            return;
-        }
         Bridge.setPref(ABK.enabled, '1');
         $('#autoBackupConfig').style.display = '';
-        toast('Auto-backup enabled');
-        // Refresh the dir/last display in case we came from cold state.
-        const fresh = await loadAutoBackupConfig();
-        refreshAutoBackupUI(fresh);
+        // No pwd prompt here — the dedicated "Set backup password" button
+        // owns it (like sync). Backups no-op until it's set.
+        const cfg = await loadAutoBackupConfig();
+        refreshAutoBackupUI(cfg);
+        toast(cfg.hasPwd ? 'Auto-backup enabled'
+                         : 'Auto-backup enabled — set a backup password to start');
     } else {
         Bridge.setPref(ABK.enabled, '');
-        // Forget the stored backup pwd so re-enabling prompts fresh —
-        // gives the user a way to change it without extra UI.
-        Bridge.setPref(ABK.pwd, '');
+        // Keep the stored pwd — re-enabling reuses it silently. Change it
+        // anytime via the dedicated "Change backup password" button.
         $('#autoBackupConfig').style.display = 'none';
         toast('Auto-backup disabled');
     }
 }
 
+// Dedicated pwd button (mirrors sync's syncSetEncPwdFlow) so the user can
+// change the backup password without the disable/re-enable dance.
+async function changeBackupPwd() {
+    if (await promptAndStoreBackupPwd()) {
+        refreshAutoBackupUI(await loadAutoBackupConfig());
+        toast('Backup password saved');
+    }
+}
+
 async function runAutoBackupNow(silent) {
     const cfg = await loadAutoBackupConfig();
     if (!cfg) return silent || toast('Bridge not available', 'error');