테마 설정 local storage 설정하도록 수정

frontend/src/utils/localThemeStorage.ts

@@ -0,0 +1,41 @@
+import type { Theme } from '../types';
+
+const STORAGE_KEY = 'gc_local_theme';
+
+const VALID: readonly Theme[] = ['dark', 'light', 'blue'];
+
+function isTheme(v: unknown): v is Theme {
+  return typeof v === 'string' && (VALID as readonly string[]).includes(v);
+}
+
+/** 이 브라우저(기기)에 저장된 UI 테마. 계정·DB와 무관 */
+export function loadLocalTheme(): Theme {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (isTheme(raw)) return raw;
+  } catch {
+    /* private mode 등 */
+  }
+  return 'dark';
+}
+
+export function saveLocalTheme(theme: Theme): void {
+  try {
+    localStorage.setItem(STORAGE_KEY, theme);
+  } catch {
+    /* quota / private mode */
+  }
+}
+
+/** localStorage에 없을 때만 DB defaultTheme을 이 기기에 1회 복사 */
+export function migrateDbThemeToLocalIfNeeded(dbTheme: string | undefined): Theme {
+  try {
+    const existing = localStorage.getItem(STORAGE_KEY);
+    if (isTheme(existing)) return existing;
+  } catch {
+    /* ignore */
+  }
+  const theme: Theme = isTheme(dbTheme) ? dbTheme : 'dark';
+  saveLocalTheme(theme);
+  return theme;
+}