refactor: move language selector into tenant email fieldset

Improved UI organization by moving the language selector into the same fieldset
as the tenant email input, since both are related to email notifications. The
language selector now only appears when email notifications are enabled.

Changes:
- Move language selector inside tenant email fieldset (no separate fieldset)
- Add label for tenant email input field
- Simplify notification language label text
- Remove unused notification-language-legend translation key
- Language selector shares visibility condition with email field

This provides a more logical grouping of related settings and reduces visual
clutter when email notifications are disabled.

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/messages/en.json

@@ -195,6 +195,7 @@
     "rent-amount-label": "Monthly rent amount",
     "rent-amount-placeholder": "enter rent amount",
     "tenant-email-legend": "TENANT EMAIL",
+    "tenant-email-label": "Send tenant notifications to e-mail address:",
     "tenant-email-placeholder": "enter tenant's email",
     "email-status": {
       "reset-button-label": "Reset",
@@ -205,8 +206,7 @@
       "verified": "this e-mail address has been verified",
       "unsubscribed": "tenant unsubscribed this address from receiving emails"
     },
-    "notification-language-legend": "NOTIFICATION EMAIL LANGUAGE",
-    "notification-language-label": "Language for automatic notification emails",
+    "notification-language-label": "Send tenant notifications in language:",
     "notification-language-option-hr": "Croatian (Hrvatski)",
     "notification-language-option-en": "English",
     "warning-missing-tenant-names": "Warning: Tenant first and last name are missing. The 2D barcode will not be displayed to the tenant when they open the shared link until both fields are filled in.",