feat: replace raw error messages with structured error codes

Backend now returns { code: ErrorCode, message: string } for all errors,
making it language-agnostic. Frontend maps codes to localised strings via
Paraglide (en/de/es), so translations live in messages/*.json.

- Add ErrorCode enum and DomainException with static factory methods
- Update GlobalExceptionHandler to return ErrorResponse(code, message)
- Replace ResponseStatusException throughout controllers/services/aspects
- Add frontend errors.ts with parseBackendError() and getErrorMessage()
- getErrorMessage() delegates to Paraglide m.error_*() functions
- Add error_* keys to messages/en.json, de.json, es.json
- Update all page.server.ts files to use the new error utilities
- Fix hardcoded localhost URLs in admin and login pages
- Fix missing baseUrl in deleteTag action

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

frontend/messages/en.json

@@ -1,4 +1,14 @@
 {
 	"$schema": "https://inlang.com/schema/inlang-message-format",
-	"hello_world": "Hello, {name} from en!"
+	"hello_world": "Hello, {name} from en!",
+	"error_document_not_found": "Document not found.",
+	"error_document_no_file": "No file is associated with this document.",
+	"error_file_not_found": "The file could not be found in storage.",
+	"error_file_upload_failed": "The file could not be uploaded.",
+	"error_user_not_found": "User not found.",
+	"error_import_already_running": "An import is already running. Please wait for it to finish.",
+	"error_unauthorized": "You are not logged in.",
+	"error_forbidden": "You do not have permission for this action.",
+	"error_validation_error": "The input is invalid.",
+	"error_internal_error": "An unexpected error occurred."
 }