feat(timer): Timer/Countdown-Widget mit Presets und Alarm

Countdown-Timer als Single-Instance-Widget mit Preset-System
(max. 5), Web Audio API Alarm und Tab-Titel-Blink bei Ablauf.
Mute-Toggle zum Stummschalten des Alarms.
Z-Index-Hierarchie für Widgets auf 100 angehoben.

src/js/app.js

@@ -19,6 +19,7 @@ async function init() {
   await migrateSticky();
   await Notes.init();
   await Calculator.init();
+  await Timer.init();
   initDataButtons();
   Store.checkQuota();
 
@@ -100,7 +101,8 @@ async function checkBackupReminder() {
     const widgetData = await Store.get('widgetStates');
     const notesData = (widgetData && Array.isArray(widgetData.notes)) ? widgetData.notes : [];
     const calcHistory = (widgetData && widgetData.calculator) ? widgetData.calculator.history || [] : [];
-    const data = { version: '1.7.0', exported: new Date().toISOString(), boards, settings, notes: notesData, calculator: calcHistory };
+    const timerPresets = (widgetData && widgetData.timer) ? widgetData.timer.presets || [] : [];
+    const data = { version: '1.7.0', exported: new Date().toISOString(), boards, settings, notes: notesData, calculator: calcHistory, timerPresets };
     const blob = new Blob([JSON.stringify(data, null, 2)], { type: 'application/json' });
     const url  = URL.createObjectURL(blob);
     const a    = document.createElement('a');