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;

src/lib/stores/preferences.svelte.js

@@ -98,7 +98,9 @@ function persistToSQLite(prefs) {
   }, 500);
 }
 
-let preferences = $state(readFromDOM());
+// Export as const so it is never reassigned — consumers hold a stable reference
+// and mutations are tracked by Svelte 5's deep reactivity.
+export const preferences = $state(readFromDOM());
 
 // Ensure data-theme and zone attributes are always present on the DOM, even
 // when there is no Tauri webview injection script (e.g. browser dev mode).
@@ -106,21 +108,19 @@ if (typeof window !== 'undefined') {
   applyToDOM(preferences);
 }
 
+/** @deprecated Use `preferences` directly — kept for backwards compatibility */
 export function getPreferences() {
   return preferences;
 }
 
 export function updatePreferences(updates) {
-  preferences = { ...preferences, ...updates };
+  Object.assign(preferences, updates);
   applyToDOM(preferences);
   persistToSQLite(preferences);
 }
 
 export function updateAccessibility(updates) {
-  preferences = {
-    ...preferences,
-    accessibility: { ...preferences.accessibility, ...updates }
-  };
+  Object.assign(preferences.accessibility, updates);
   applyToDOM(preferences);
   persistToSQLite(preferences);
 }