Allow check intervals as low as 10 seconds

- Switch from Chrome alarms (30s min) to setTimeout loops
- Config now uses checkIntervalSeconds instead of minutes
- Default: 15 seconds, minimum: 10 seconds
- Prevents overlapping checks with isChecking flag
- Updated popup UI to show seconds

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

chrome-extension/popup.html

@@ -214,8 +214,9 @@
 
   <div class="row">
     <div>
-      <label>Check Interval (min)</label>
-      <input type="number" id="interval" min="1" max="60" value="1">
+      <label>Check Interval (seconds)</label>
+      <input type="number" id="interval" min="10" max="300" value="15">
+      <small>Min 10 sec. Recommended: 15-30 sec</small>
     </div>
   </div>