fix(web): widget cache-busting, dead frontend code, and dependency pins (#656)

- Plugin-supplied widgets load as /static/plugin-widgets/...js?v=<plugin
  version>, so an update isn't hidden behind the year-long immutable cache.
- Fire-and-forget loadInstalledPlugins() calls catch the rejection it has
  already reported, so the global handler no longer adds a second toast.
- Timezone picker renders again when the General partial is re-injected.
- Remove dead code: executePluginAction's six plugin-id fallbacks and
  [DEBUG] logging, window.currentPluginConfig and every read of it, the
  file-upload JSON delete branch, unused PluginAPI / PluginInstallManager /
  PluginStateManager helpers, loadPluginWidgetsFromManifest, the stale
  install_manager.js and LEDVisibility fallbacks, error_handler.js's global
  escapeHtml, 13 unused CSS rules, and stale comments/no-op returns.
- pytz < 2027, psutil < 7 in requirements-test.txt, pytest-cov < 8.
- Pin anthropics/claude-code-action to the commit v1 resolves to.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/templates/v3/partials/display.html

@@ -1044,11 +1044,7 @@ document.getElementById('brightness').addEventListener('input', function() {
     }
     function startSyncPolling() {
         updateSyncUI();
-        if (window.LEDVisibility) {
-            window.LEDVisibility.onActive('display', startSyncTimer, stopSyncTimer, 'display-sync-status');
-        } else if (!window.syncStatusInterval) {
-            window.syncStatusInterval = setInterval(pollSyncStatus, 5000);
-        }
+        window.LEDVisibility.onActive('display', startSyncTimer, stopSyncTimer, 'display-sync-status');
     }
 
     if (document.readyState === 'loading') {