Add NextSnap PWA with photo gallery viewer and continuous capture

Offline-first photo capture app for Nextcloud with:
- Camera capture with continuous mode (auto-reopens after each photo)
- File browser with fullscreen image gallery, swipe navigation, and rename
- Upload queue with background sync engine
- Admin panel for Nextcloud user management
- Service worker for offline-first caching (v13)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/static/js/auth.js

@@ -0,0 +1,160 @@
+// NextSnap - Authentication logic
+'use strict';
+
+const Auth = {
+    init() {
+        const loginForm = document.getElementById('login-form');
+        if (loginForm) {
+            loginForm.addEventListener('submit', this.handleLogin.bind(this));
+            
+            // Add input validation
+            const usernameInput = document.getElementById('username');
+            const passwordInput = document.getElementById('password');
+            
+            if (usernameInput) {
+                usernameInput.addEventListener('blur', () => this.validateUsername());
+                usernameInput.addEventListener('input', () => this.clearFieldError('username'));
+            }
+            
+            if (passwordInput) {
+                passwordInput.addEventListener('input', () => this.clearFieldError('password'));
+            }
+        }
+    },
+    
+    validateUsername() {
+        const username = document.getElementById('username').value.trim();
+        const usernameError = document.getElementById('username-error');
+        const usernameInput = document.getElementById('username');
+        
+        if (!username) {
+            usernameError.textContent = 'Username is required';
+            usernameInput.classList.add('error');
+            return false;
+        }
+        
+        if (username.length < 2) {
+            usernameError.textContent = 'Username must be at least 2 characters';
+            usernameInput.classList.add('error');
+            return false;
+        }
+        
+        usernameError.textContent = '';
+        usernameInput.classList.remove('error');
+        return true;
+    },
+    
+    validatePassword() {
+        const password = document.getElementById('password').value;
+        const passwordError = document.getElementById('password-error');
+        const passwordInput = document.getElementById('password');
+        
+        if (!password) {
+            passwordError.textContent = 'Password is required';
+            passwordInput.classList.add('error');
+            return false;
+        }
+        
+        passwordError.textContent = '';
+        passwordInput.classList.remove('error');
+        return true;
+    },
+    
+    clearFieldError(field) {
+        const errorElement = document.getElementById(`${field}-error`);
+        const inputElement = document.getElementById(field);
+        if (errorElement) errorElement.textContent = '';
+        if (inputElement) inputElement.classList.remove('error');
+    },
+    
+    async handleLogin(event) {
+        event.preventDefault();
+        
+        // Validate fields
+        const usernameValid = this.validateUsername();
+        const passwordValid = this.validatePassword();
+        
+        if (!usernameValid || !passwordValid) {
+            return;
+        }
+        
+        const username = document.getElementById('username').value.trim();
+        const password = document.getElementById('password').value;
+        const errorMessage = document.getElementById('error-message');
+        const loginBtn = document.getElementById('login-btn');
+        const loginBtnText = document.getElementById('login-btn-text');
+        const loginBtnLoading = document.getElementById('login-btn-loading');
+        
+        // Clear previous error
+        errorMessage.classList.add('hidden');
+        errorMessage.textContent = '';
+        
+        // Disable button and show loading state
+        loginBtn.disabled = true;
+        loginBtnText.classList.add('hidden');
+        loginBtnLoading.classList.remove('hidden');
+        
+        try {
+            const response = await fetch('/api/auth/login', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json'
+                },
+                body: JSON.stringify({ username, password })
+            });
+            
+            const data = await response.json();
+            
+            if (response.ok && data.success) {
+                // Login successful - redirect to capture page
+                window.location.href = '/capture';
+            } else {
+                // Login failed - show error
+                const errorText = data.error || 'Login failed. Please check your credentials and try again.';
+                errorMessage.textContent = errorText;
+                errorMessage.classList.remove('hidden');
+                
+                // Focus back on username for retry
+                document.getElementById('username').focus();
+            }
+        } catch (error) {
+            console.error('Login error:', error);
+            errorMessage.textContent = 'Network error. Please check your connection and try again.';
+            errorMessage.classList.remove('hidden');
+        } finally {
+            // Re-enable button and restore normal state
+            loginBtn.disabled = false;
+            loginBtnText.classList.remove('hidden');
+            loginBtnLoading.classList.add('hidden');
+        }
+    },
+    
+    async checkStatus() {
+        try {
+            const response = await fetch('/api/auth/status');
+            const data = await response.json();
+            return data.authenticated ? data : null;
+        } catch (error) {
+            console.error('Auth status check failed:', error);
+            return null;
+        }
+    },
+    
+    async logout() {
+        try {
+            await fetch('/api/auth/logout', { method: 'POST' });
+            window.location.href = '/';
+        } catch (error) {
+            console.error('Logout error:', error);
+            // Redirect anyway
+            window.location.href = '/';
+        }
+    }
+};
+
+// Initialize when DOM is ready
+if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', () => Auth.init());
+} else {
+    Auth.init();
+}