fix: remove orphaned functions, add for auto-load columns

Clean up remaining orphaned references after tab extraction:
- Remove loadColumnsForDatasource, searchDatasources, selectDatasource
  etc. from page (moved to ConfigTabForm)
- Add  on datasourceId in ConfigTabForm for auto-loading
  columns when parent sets datasourceId (loadJob)
- Fix stray braces, remove dead isVirtual function
- Page now 779 lines (down from 1519, -49%)

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

@@ -421,95 +421,6 @@
     }
   }
 
-  /** @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 {
-      const result = await fetchDatasourceColumns(parseInt(datasourceId), environmentId);
-      if (result) {
-        availableColumns = result.columns || [];
-        databaseDialect = result.database_dialect || '';
-        virtualColumns = availableColumns.filter(c => c.is_physical === false).map(c => c.name);
-      }
-    } catch (err) {
-      uxState = 'datasource_unavailable';
-      addToast(`${_('common.error')}: ${err?.message}`, 'error');
-    } finally {
-      isColumnsLoading = false;
-    }
-  }
-
-  /** Load all datasets (or search) */
-  async function searchDatasources(searchTerm = '') {
-    if (!environmentId) return;
-    datasourceLoading = true;
-    try {
-      datasourceList = await fetchDatasources(environmentId, searchTerm);
-      showDatasourceDropdown = true;
-    } catch (err) {
-      datasourceList = [];
-    } finally {
-      datasourceLoading = false;
-    }
-  }
-
-  async function handleDatasourceSearch() {
-    await searchDatasources(datasourceSearch);
-  }
-
-  /** Select a dataset from the dropdown */
-  function selectDatasource(ds) {
-    datasourceId = String(ds.id);
-    datasourceSearch = `${ds.table_name} (${ds.database_name} · ${ds.database_dialect})`;
-    showDatasourceDropdown = false;
-    availableColumns = [];
-    virtualColumns = [];
-    sourceKeyCols = [];
-    targetKeyCols = [];
-    translationColumn = '';
-    targetColumn = '';
-    targetLanguageColumn = '';
-    targetSourceColumn = '';
-    targetSourceLanguageColumn = '';
-    contextColumns = [];
-    databaseDialect = ds.database_dialect || '';
-    // Auto-set target database ID from the selected datasource's database
-    if (ds.database_id) {
-      targetDatabaseId = String(ds.database_id);
-    }
-    if (datasourceId && environmentId) {
-      loadColumnsForDatasource();
-    }
-  }
-
-  /** @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 = [];
-
-    if (val && environmentId) {
-      await loadColumnsForDatasource();
-    }
-  }
-
   async function loadDatabases() {
     if (!environmentId) {
       databases = [];
@@ -533,38 +444,6 @@
     targetDatabaseId = '';
     await loadDatabases();
   }
-  }
-
-  /** @param {string} col */
-  function toggleContextColumn(col) {
-    if (contextColumns.includes(col)) {
-      contextColumns = contextColumns.filter(c => c !== col);
-    } else {
-      contextColumns = [...contextColumns, col];
-    }
-  }
-
-  /** @param {string} col */
-  function toggleSourceKeyCol(col) {
-    const idx = sourceKeyCols.indexOf(col);
-    if (idx >= 0) {
-      sourceKeyCols = sourceKeyCols.filter((_, i) => i !== idx);
-      // Also remove corresponding target key
-      if (targetKeyCols[idx]) {
-        targetKeyCols = targetKeyCols.filter((_, i) => i !== idx);
-      }
-    } else {
-      sourceKeyCols = [...sourceKeyCols, col];
-      targetKeyCols = [...targetKeyCols, ''];
-    }
-  }
-
-  /** @param {number} idx @param {string} val */
-  function updateTargetKeyCol(idx, val) {
-    const updated = [...targetKeyCols];
-    updated[idx] = val;
-    targetKeyCols = updated;
-  }
 
   /** @returns {string[]} */
   function validate() {
@@ -662,11 +541,6 @@
     }
   }
 
-  /** @param {string} col */
-  function isVirtual(col) {
-    return virtualColumnNames.includes(col);
-  }
-
   function getJobStatusLabel(value) {
     const labels = {
       DRAFT: $t.translate?.config?.status_draft || 'Draft',