fix(preferences): use Object.assign mutation to prevent infinite effect loops

Replacing the preferences object (spread reassignment) caused all consumers'
prefs references to go stale — the loop guard read the old value forever.
Svelte 5 shared state pattern requires a stable const object with property
mutations, not reassignment. Also removes duplicate theme sync $effect from
SettingsPage (already handled in +layout.svelte).

src/lib/pages/SettingsPage.svelte

@@ -281,14 +281,6 @@
     saveSettings();
   });
 
-  // Sync theme setting to preferences store
-  $effect(() => {
-    const map = { Dark: 'dark', Light: 'light', System: 'system' };
-    const mapped = map[settings.theme] || 'dark';
-    if (prefs.theme !== mapped) {
-      updatePreferences({ theme: mapped });
-    }
-  });
 
   $effect(() => {
     settings.engine;