feat(settings): split Autofill into its own tab

Extracted the autofill toggle + hotkey combos out of the Security section into
their own .slideover-field labelled "Autofill", added an Autofill tab + map
entry. 5 tabs now.

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

js/app.js

@@ -7813,6 +7813,7 @@ function closeSettings() {
 const SETTINGS_TAB_OF = {
     'Appearance': 'general',
     'Security': 'security', 'Clipboard privacy': 'security',
+    'Autofill': 'autofill',
     'Account': 'account',
     'PIN unlock': 'account', 'Quick unlock': 'account', 'Recovery key': 'account',
     'Export': 'sync', 'Sync (WebDAV)': 'sync', 'Auto-backup': 'sync', 'Import': 'sync',