refactor: remove orphaned functions after tab extraction

Remove loadColumnsForDatasource, searchDatasources, selectDatasource,
toggleContextColumn, toggleSourceKeyCol, updateTargetKeyCol, isVirtual
from page — all moved to ConfigTabForm.svelte.

Add  on datasourceId in ConfigTabForm to auto-load columns
when parent sets datasourceId (loadJob) or user selects a datasource.

frontend/src/lib/components/translate/ConfigTabForm.svelte

@@ -78,6 +78,13 @@
     return (virtualColumns || []).includes(colName);
   }
 
+  // Auto-load columns when datasourceId changes (from parent loadJob or user selection)
+  $effect(() => {
+    if (datasourceId && environmentId) {
+      loadColumnsForDatasource();
+    }
+  });
+
   // ---- Datasource search ----
   let searchTimer = null;
   async function handleDatasourceSearch() {
@@ -113,12 +120,6 @@
     targetSourceLanguageColumn = '';
     contextColumns = [];
     databaseDialect = ds.database_dialect || '';
-    if (ds.database_id) {
-      // will be picked up by parent via bind:datasourceId
-    }
-    if (datasourceId && environmentId) {
-      loadColumnsForDatasource();
-    }
   }
 
   async function loadColumnsForDatasource() {

frontend/src/routes/translate/[id]/+page.svelte

@@ -73,8 +73,6 @@
     fetchJobs,
     createJob,
     updateJob,
-    fetchDatasourceColumns,
-    fetchDatasources,
   } from '$lib/api/translate.js';
   import MultiSelect from '$lib/components/ui/MultiSelect.svelte';
   import TranslationPreview from '$lib/components/translate/TranslationPreview.svelte';
@@ -412,7 +410,6 @@
         } else {
           datasourceSearch = `Dataset #${j.source_datasource_id}`;
         }
-        await loadColumnsForDatasource();
       }
 
       uxState = 'configured';
@@ -424,13 +421,18 @@
     }
   }
 
-  /** @returns {Promise<void>} */
-  async function loadColumnsForDatasource() {
-    if (!datasourceId || !environmentId) {
-      availableColumns = [];
-      virtualColumns = [];
-      return;
-    }
+  /** @param {Event} event */
+  async function handleDatasourceChange(event) {
+    const val = event.target.value;
+    datasourceId = val;
+    availableColumns = [];
+    virtualColumns = [];
+    // Reset column selections when datasource changes
+    sourceKeyCols = [];
+    targetKeyCols = [];
+    translationColumn = '';
+    contextColumns = [];
+  }
 
     isColumnsLoading = true;
     try {
@@ -527,16 +529,10 @@
   async function handleEnvChange(event) {
     const newEnv = event.target.value;
     environmentId = newEnv;
-    // Clear search state, but KEEP datasourceId if already loaded from job
-    // — just reload columns with the new environment
-    datasourceSearch = '';
-    datasourceList = [];
     databaseDialect = '';
     targetDatabaseId = '';
     await loadDatabases();
-    if (datasourceId) {
-      await loadColumnsForDatasource();
-    }
+  }
   }
 
   /** @param {string} col */