feat(auth): implement user authentication and profile management

- Added authentication context and provider to manage user state.
- Created login and registration forms with validation and error handling.
- Implemented user profile component for updating user information and changing passwords.
- Introduced user settings in the database for notification preferences.
- Updated translations for authentication-related strings in English and German.
- Enhanced styles for authentication components and user profile.
- Added middleware for optional and required authentication checks.

.env.example

@@ -11,12 +11,25 @@ LOG_LEVEL=info
 # Timezone for scheduled reminders (e.g., Europe/Berlin, America/New_York)
 TZ=Europe/Berlin
 
-# Auth - CHANGE THESE! Generate with: openssl rand -hex 32
-JWT_SECRET=CHANGE_ME_generate_with_openssl_rand_hex_32
-REFRESH_SECRET=CHANGE_ME_generate_with_openssl_rand_hex_32
-COOKIE_SECRET=CHANGE_ME_generate_with_openssl_rand_hex_32
+# =============================================================================
+# Authentication (optional - disabled by default for easy setup)
+# =============================================================================
+# Enable authentication (default: false = open access)
+AUTH_ENABLED=false
 
-# SMTP (optional - for email notifications)
+# Allow new user registrations (auto-enabled when no users exist)
+# REGISTRATION_ENABLED=false
+
+# Disable local auth (for SSO-only setups in Phase 2)
+# DISABLE_LOCAL_AUTH=false
+
+# JWT Secrets - REQUIRED when AUTH_ENABLED=true
+# Generate with: openssl rand -hex 32
+# JWT_SECRET=
+# REFRESH_SECRET=
+# COOKIE_SECRET=
+
+# SMTP (optional - for email notifications and password reset)
 SMTP_HOST=
 SMTP_PORT=587
 SMTP_USER=