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>

css/style.css

@@ -2198,10 +2198,12 @@ body[data-editor-position="center"]:has(#settingsPanel.is-open)::before {
     flex: 0 0 auto; border-right: 1px solid var(--border); overflow-y: auto;
 }
 .settings-tab {
+    display: flex; align-items: center; gap: 8px;
     text-align: left; width: 100%; padding: 7px 12px; font-size: 12px; cursor: pointer;
     color: var(--text-dim); background: transparent; border: 1px solid transparent;
     border-radius: 6px; white-space: nowrap;
 }
+.settings-tab svg { width: 15px; height: 15px; flex: 0 0 auto; }
 .settings-tab:hover { background: var(--bg-elev-2); }
 .settings-tab.is-active { color: var(--accent-fg); background: var(--accent); border-color: var(--accent); }
 .slideover-field.is-tab-hidden { display: none; }