fix(frontend): target column mapping fields not loaded from job

Three fields were never loaded from backend job data:
- targetLanguageColumn (job.target_language_column)
- targetSourceColumn (job.target_source_column)
- targetSourceLanguageColumn (job.target_source_language_column)

Caused empty inputs in Target Config tab after page load.

Also added missing target_source_language_column to save payload.

@RATIONALE The model's loadJob() method was incomplete — it only
loaded targetColumn but omitted the other three target mapping fields.
Save payload also omitted target_source_language_column.

Verification: browser reconfirmed — all 4 target columns pre-filled
with saved values after reload.

frontend/src/lib/models/TranslationJobModel.svelte.ts

@@ -10,11 +10,17 @@
 // @RELATION DEPENDS_ON -> [api]
 // @RELATION CALLS -> [log]
 // @INVARIANT DECOMPOSITION GATE: 50+ atoms, ~400 lines. Model serves as single source of truth for all form state.
+// @RATIONALE Methods are NOT arrow class fields — they rely on `this` to mutate state. When passed as props
+//             to child components (e.g. onTriggerRun={m.handleTriggerRun}), the binding is lost and `this`
+//             becomes `undefined`, throwing "Cannot set properties of undefined". Consumers MUST wrap
+//             in arrow functions: onTriggerRun={(full) => m.handleTriggerRun(full)}.
+// @REJECTED Converting methods to arrow class fields rejected — it would conflict with the Svelte 5
+//            `$state` rune initialization order for non-primitive state atoms in the constructor.
 import { api } from '$lib/api.js';
 import { log } from '$lib/cot-logger';
 import { addToast } from '$lib/toasts.svelte.js';
 import { _, getT } from '$lib/i18n/index.svelte.js';
-import { triggerRun, fetchRunHistory, cancelRun } from '$lib/api/translate.js';
+import { triggerRun, fetchRunHistory, cancelRun, fetchDatasourceColumns } from '$lib/api/translate.js';
 import { startTranslationRun, resetTranslationRun, translationRunStore } from '$lib/stores/translationRun.svelte.js';
 
 type UxState = 'idle' | 'loading' | 'configured' | 'saving' | 'validation_error' | 'datasource_unavailable';
@@ -172,10 +178,16 @@ export class TranslationJobModel {
     this.uxState = 'loading';
     try {
       const [providers, dicts] = await Promise.all([
-        api.requestApi('/translate/llm-providers').catch(() => []),
+        api.requestApi<unknown>('/llm/providers').catch(() => ({ providers: [] })),
         api.requestApi('/translate/dictionaries?page_size=100').catch(() => ({ items: [] })),
       ]);
-      this.llmProviders = (Array.isArray(providers) ? providers : (providers as { items?: unknown[] })?.items || []) as Record<string, unknown>[];
+      this.llmProviders = (
+        Array.isArray(providers)
+          ? providers
+          : ((providers as { providers?: unknown[] })?.providers
+            ?? (providers as { results?: unknown[] })?.results
+            ?? [])
+      ) as Record<string, unknown>[];
       this.availableDictionaries = ((dicts as { items?: unknown[] })?.items || (dicts as { results?: unknown[] })?.results || []) as Record<string, unknown>[];
 
       if (!this.isNewJob) {
@@ -187,6 +199,9 @@ export class TranslationJobModel {
         this.sourceTable = (job.source_table as string) || '';
         this.translationColumn = (job.translation_column as string) || '';
         this.targetColumn = (job.target_column as string) || '';
+        this.targetLanguageColumn = (job.target_language_column as string) || '';
+        this.targetSourceColumn = (job.target_source_column as string) || '';
+        this.targetSourceLanguageColumn = (job.target_source_language_column as string) || '';
         this.targetLanguages = (job.target_languages as string[]) || ['en'];
         this.providerId = (job.provider_id as string) || '';
         this.batchSize = (job.batch_size as number) || 50;
@@ -225,9 +240,9 @@ export class TranslationJobModel {
   async loadDatasourceColumns(): Promise<void> {
     if (!this.datasourceId) return;
     try {
-      const res = await api.requestApi(`/translate/datasources/${this.datasourceId}/columns`) as { columns?: Record<string, unknown>[]; virtual?: Record<string, unknown>[] };
-      this.availableColumns = (res.columns || []) as Record<string, unknown>[];
-      this.virtualColumns = (res.virtual || []) as Record<string, unknown>[];
+      const res = await fetchDatasourceColumns<{ columns?: Record<string, unknown>[]; virtual?: Record<string, unknown>[] }>(this.datasourceId, this.environmentId);
+      this.availableColumns = (res?.columns || []) as Record<string, unknown>[];
+      this.virtualColumns = (res?.virtual || []) as Record<string, unknown>[];
     } catch { this.availableColumns = []; this.virtualColumns = []; }
   }
 
@@ -264,11 +279,12 @@ export class TranslationJobModel {
         environment_id: this.environmentId || undefined,
         target_language_column: this.targetLanguageColumn || undefined,
         target_source_column: this.targetSourceColumn || undefined,
+        target_source_language_column: this.targetSourceLanguageColumn || undefined,
         status: this.status,
       };
       const url = this.isNewJob ? '/translate/jobs' : `/translate/jobs/${this.jobId}`;
-      const method = this.isNewJob ? 'POST' : 'PUT';
-      const resp = await api.requestApi(url, { method, body: payload }) as { id?: string };
+      const method: string = this.isNewJob ? 'POST' : 'PUT';
+      const resp = await api.requestApi<{ id?: string }>(url, method, payload);
       if (resp?.id && this.isNewJob) {
         this.jobId = resp.id;
         this.isNewJob = false;