fix: auto-sync token persistence + better status display

Auto-sync now:
- Persists Gist token in the auto-sync config so it survives
  page reloads (previously only read from the input field)
- Saves token when the Gist token field changes (not just on toggle)
- Validates that credentials exist before enabling
- Shows current status: method, interval, last push/pull times
- Updated UI description to clarify it works in all browsers

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/options/options.html

@@ -278,9 +278,11 @@
 
       <!-- Auto Sync -->
       <div style="margin-top:20px;padding-top:16px;border-top:1px solid #e5e7eb">
-        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Auto Sync — background polling</h3>
+        <h3 style="font-size:13px;font-weight:600;margin:0 0 4px">Auto Sync — works in all browsers</h3>
         <p class="section-desc" style="margin-bottom:8px">
-          Automatically push and pull settings on an interval. Uses the Gist or URL method configured above.
+          Automatically push and pull settings in the background on a schedule.
+          Uses GitHub Gist or Custom URL — configure one of those above first, then enable auto sync here.
+          Changes you make locally are pushed immediately; remote changes are pulled on the interval.
         </p>
         <div style="display:flex;gap:12px;flex-wrap:wrap;align-items:center;margin-bottom:8px">
           <label class="toggle" title="Enable auto sync">