fix: restore frontend source code (src/web/) that was accidentally deleted

Frontend source is needed for development/building the SPA.
Only Node.js runtime server code should be removed.

src/web/appLocalState.ts

@@ -0,0 +1,28 @@
+import { clearAuthSession } from './authSession.js';
+
+export const THEME_MODE_STORAGE_KEY = 'theme_mode';
+export const LEGACY_THEME_STORAGE_KEY = 'theme';
+export const USER_PROFILE_STORAGE_KEY = 'user_profile';
+export const FIRST_USE_DOC_REMINDER_KEY = 'boosapi_first_use_docs_reminder_seen_v1';
+
+type StorageLike = {
+  getItem?: (key: string) => string | null;
+  setItem?: (key: string, value: string) => void;
+  removeItem: (key: string) => void;
+};
+
+function resolveStorage(storage?: StorageLike | null): StorageLike | null {
+  if (storage) return storage;
+  if (typeof localStorage !== 'undefined') return localStorage;
+  return null;
+}
+
+export function clearAppInstallationState(storage?: StorageLike | null): void {
+  const target = resolveStorage(storage);
+  if (!target) return;
+  clearAuthSession(target as never);
+  target.removeItem(THEME_MODE_STORAGE_KEY);
+  target.removeItem(LEGACY_THEME_STORAGE_KEY);
+  target.removeItem(USER_PROFILE_STORAGE_KEY);
+  target.removeItem(FIRST_USE_DOC_REMINDER_KEY);
+}