fix(ocr): replace IllegalStateException with DomainException.internal in triggerManualSenderTraining

Ensures the unexpected-state path produces a structured JSON error response
instead of an unmapped 500 RuntimeException. Adds OCR_TRAINING_CONFLICT
ErrorCode and mirrors it in the frontend errors.ts. Adds coverage tests for
getAllSenderModels() and runSenderTraining().

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

frontend/src/lib/errors.ts

@@ -27,6 +27,7 @@ export type ErrorCode =
 	| 'OCR_DOCUMENT_NOT_UPLOADED'
 	| 'OCR_PROCESSING_FAILED'
 	| 'TRAINING_ALREADY_RUNNING'
+	| 'OCR_TRAINING_CONFLICT'
 	| 'INVALID_TAG_COLOR'
 	| 'TAG_CYCLE_DETECTED'
 	| 'TAG_NOT_FOUND'
@@ -105,6 +106,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_ocr_processing_failed();
 		case 'TRAINING_ALREADY_RUNNING':
 			return m.error_training_already_running();
+		case 'OCR_TRAINING_CONFLICT':
+			return m.error_internal_error();
 		case 'INVALID_TAG_COLOR':
 			return m.error_invalid_tag_color();
 		case 'TAG_CYCLE_DETECTED':