refactor(web): safeLocalStorage helper for store persistence (#375)

Three nearly-identical try/catch wrappers across theme + player stores
collapse to read()/write()/remove() in lib/util/safeLocalStorage.ts.
Sets the pattern for future stores. Caller still does parse/serialize
since the existing call sites store strings (theme preference, volume
number) — no JSON wrapper needed yet.

persisted.ts left alone — its JSON-payload + per-key-suffix shape is
distinct enough to keep self-contained.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/stores/theme.svelte.ts

@@ -1,16 +1,13 @@
+import * as storage from '$lib/util/safeLocalStorage';
+
 const STORAGE_KEY = 'minstrel:theme';
 
 export type ThemePreference = 'dark' | 'light' | 'system';
 export type ResolvedTheme = 'dark' | 'light';
 
 function readPref(): ThemePreference {
-  if (typeof localStorage === 'undefined') return 'system';
-  try {
-    const v = localStorage.getItem(STORAGE_KEY);
-    return v === 'dark' || v === 'light' || v === 'system' ? v : 'system';
-  } catch {
-    return 'system';
-  }
+  const v = storage.read(STORAGE_KEY);
+  return v === 'dark' || v === 'light' || v === 'system' ? v : 'system';
 }
 
 function systemResolved(): ResolvedTheme {
@@ -51,9 +48,7 @@ export const resolvedTheme = {
 export function setTheme(pref: ThemePreference): void {
   _theme = pref;
   _resolved = resolve(pref);
-  if (typeof localStorage !== 'undefined') {
-    try { localStorage.setItem(STORAGE_KEY, pref); } catch { /* ignore quota / unavailable */ }
-  }
+  storage.write(STORAGE_KEY, pref);
   applyToDOM(_resolved);
 }