Fix translation issues in event creation and detail pages

- Add missing translation keys for event creation form
- Replace all hardcoded strings with translation keys in CreateEventPage
- Add validation error message translations
- Add event types and color theme translations
- Add errors and validation sections to translation files
- Update German translations with corresponding keys

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/i18n/locales/en.json

@@ -28,7 +28,8 @@
     "refresh": "Refresh",
     "preview": "Preview",
     "processing": "Processing...",
-    "upload": "Upload"
+    "upload": "Upload",
+    "days": "days"
   },
   "upload": {
     "photoCategory": "Photo Category",
@@ -165,6 +166,30 @@
     "createEvent": "Create Event",
     "createNewEvent": "Create New Event",
     "setupNewGallery": "Set up a new photo gallery for your event",
+    "createNewEventSubtitle": "Set up a new photo gallery for your event",
+    "eventNamePlaceholder": "e.g., John & Jane's Wedding",
+    "welcomeMessageOptional": "Welcome Message (Optional)",
+    "welcomeMessagePlaceholder": "Welcome to our special day! Feel free to download and share these memories...",
+    "hostEmailPlaceholder": "host@example.com",
+    "adminEmailPlaceholder": "admin@example.com",
+    "securityAndAccess": "Security & Access",
+    "enterPassword": "Enter password",
+    "confirmPasswordPlaceholder": "Confirm password",
+    "galleryExpiresOn": "Gallery will expire on {{date}}",
+    "guestsWillReceiveWarning": "Guests will receive a warning email 7 days before expiration.",
+    "types": {
+      "wedding": "Wedding",
+      "birthday": "Birthday",
+      "corporate": "Corporate",
+      "other": "Other"
+    },
+    "themes": {
+      "default": "Default",
+      "oceanBlue": "Ocean Blue",
+      "royalPurple": "Royal Purple",
+      "roseGold": "Rose Gold",
+      "sunsetAmber": "Sunset Amber"
+    },
     "eventDetails": "Event Details",
     "eventName": "Event Name",
     "eventType": "Event Type",
@@ -548,7 +573,20 @@
     "goToHomepage": "Go to Homepage",
     "errorDetails": "Error Details",
     "requiredFields": "Please fill in all required fields",
-    "enterTestEmail": "Please enter a test email address"
+    "enterTestEmail": "Please enter a test email address",
+    "failedToCreateEvent": "Failed to create event",
+    "networkError": "Network error. Please check your connection and try again.",
+    "sessionExpired": "Session expired. Please login again."
+  },
+  "validation": {
+    "eventNameRequired": "Event name is required",
+    "hostEmailRequired": "Host email is required",
+    "adminEmailRequired": "Admin email is required",
+    "invalidEmailFormat": "Invalid email format",
+    "passwordRequired": "Password is required",
+    "passwordMinLength": "Password must be at least 6 characters",
+    "passwordsDoNotMatch": "Passwords do not match",
+    "expirationRange": "Expiration must be between 1 and 365 days"
   },
   "legal": {
     "impressum": "Legal Notice",