feat: implement user authentication with login and register

- Add login screen with email/username support
- Add register screen with email validation
- Implement AuthStore with expo-secure-store (native) / localStorage (web)
- Add X-User-Id header authentication (simple auth without JWT)
- Rename displayName to userName across codebase
- Add findByUserName() to UserRepository
- Check for existing email AND username on registration
- Add AuthButton component with shadow effect
- Add logout button to Header
- Add hash-password.js utility script for manual password resets
- Update CORS to allow X-User-Id header

apps/client/src/stores/AuthStore.ts

@@ -1,26 +1,69 @@
 import { create } from "zustand";
+import { Platform } from "react-native";
 import { User } from "@caldav/shared";
+import * as SecureStore from "expo-secure-store";
+
+const USER_STORAGE_KEY = "auth_user";
+
+// SecureStore doesn't work on web, use localStorage as fallback
+const storage = {
+  async setItem(key: string, value: string): Promise<void> {
+    if (Platform.OS === "web") {
+      localStorage.setItem(key, value);
+    } else {
+      await SecureStore.setItemAsync(key, value);
+    }
+  },
+  async getItem(key: string): Promise<string | null> {
+    if (Platform.OS === "web") {
+      return localStorage.getItem(key);
+    }
+    return SecureStore.getItemAsync(key);
+  },
+  async deleteItem(key: string): Promise<void> {
+    if (Platform.OS === "web") {
+      localStorage.removeItem(key);
+    } else {
+      await SecureStore.deleteItemAsync(key);
+    }
+  },
+};
 
 interface AuthState {
   user: User | null;
-  token: string | null;
   isAuthenticated: boolean;
-  login: (user: User, token: string) => void;
-  logout: () => void;
-  setToken: (token: string) => void;
+  isLoading: boolean;
+  login: (user: User) => Promise<void>;
+  logout: () => Promise<void>;
+  loadStoredUser: () => Promise<void>;
 }
 
 export const useAuthStore = create<AuthState>((set) => ({
   user: null,
-  token: null,
   isAuthenticated: false,
-  login: (_user: User, _token: string) => {
-    throw new Error("Not implemented");
+  isLoading: true,
+
+  login: async (user: User) => {
+    await storage.setItem(USER_STORAGE_KEY, JSON.stringify(user));
+    set({ user, isAuthenticated: true });
   },
-  logout: () => {
-    throw new Error("Not implemented");
+
+  logout: async () => {
+    await storage.deleteItem(USER_STORAGE_KEY);
+    set({ user: null, isAuthenticated: false });
   },
-  setToken: (_token: string) => {
-    throw new Error("Not implemented");
+
+  loadStoredUser: async () => {
+    try {
+      const stored = await storage.getItem(USER_STORAGE_KEY);
+      if (stored) {
+        const user = JSON.parse(stored) as User;
+        set({ user, isAuthenticated: true, isLoading: false });
+      } else {
+        set({ isLoading: false });
+      }
+    } catch {
+      set({ isLoading: false });
+    }
   },
 }));