feat: add language selector for tenant notification emails

Added ability to select the language for automatic notification emails sent to
tenants. Users can choose between Croatian (hr) and English (en). If not set,
defaults to the current UI language.

Changes:
- Add tenantEmailLanguage field to BillingLocation type (shared-code)
- Add language selector fieldset in LocationEditForm below email settings
- Add Zod validation for tenantEmailLanguage in locationActions
- Include field in all database insert and update operations
- Default to current locale if not explicitly set
- Add translation labels for language selector (EN/HR)

This allows tenants to receive bills and notifications in their preferred language
regardless of the landlord's UI language preference.

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

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

web-app/messages/en.json

@@ -205,6 +205,10 @@
       "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-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.",
     "save-button": "Save",
     "cancel-button": "Cancel",