feat(sqlserver): enable table structure editing for existing columns and comments

- Enable renameColumn, alterExistingColumn, alterType, alterNullability,
  alterDefault, comment, and indexComment capabilities for SQL Server
- Implement build_sqlserver_existing_column_sql() with sp_rename for column
  rename, ALTER TABLE ALTER COLUMN for type/nullability changes, and dynamic
  SQL for default constraint management
- Add sp_addextendedproperty/sp_dropextendedproperty DDL generation for table,
  column, and index comments
- Fetch table and column comments from sys.extended_properties via OUTER APPLY
  in list_tables, list_objects, and get_columns SQL queries
- Fix table comment placeholder text (was showing column comment placeholder)

apps/desktop/src/components/structure/TableStructureEditor.vue

@@ -464,7 +464,7 @@ watch(
       <label class="shrink-0 text-[11px] font-medium text-muted-foreground">{{ t("structureEditor.comment") }}</label>
       <Input
         v-model="tableComment"
-        :placeholder="t('structureEditor.commentPlaceholder')"
+        :placeholder="t('structureEditor.tableCommentPlaceholder')"
         class="h-6 max-w-[320px] text-[11px]"
       />
     </div>

apps/desktop/src/i18n/locales/en.ts

@@ -973,6 +973,7 @@ export default {
     comment: "Comment",
     editComment: "Edit comment",
     commentPlaceholder: "Enter column comment...",
+    tableCommentPlaceholder: "Enter table comment...",
     actions: "Actions",
     indexName: "Index",
     indexColumns: "Columns",

apps/desktop/src/i18n/locales/es.ts

@@ -867,6 +867,7 @@ export default {
     comment: "Comentario",
     editComment: "Editar comentario",
     commentPlaceholder: "Ingresar comentario de columna...",
+    tableCommentPlaceholder: "Ingresar comentario de tabla...",
     actions: "Acciones",
     indexName: "Índice",
     indexColumns: "Columnas",

apps/desktop/src/i18n/locales/zh-CN.ts

@@ -951,6 +951,7 @@ export default {
     comment: "注释",
     editComment: "编辑注释",
     commentPlaceholder: "输入字段注释...",
+    tableCommentPlaceholder: "输入表注释...",
     actions: "操作",
     indexName: "索引名",
     indexColumns: "字段列表",

apps/desktop/src/lib/tableStructureCapabilities.ts

@@ -139,12 +139,19 @@ const sqlserverCapabilities = capabilities({
   createTable: true,
   addColumn: true,
   dropColumn: true,
+  renameColumn: true,
+  alterExistingColumn: true,
+  alterType: true,
+  alterNullability: true,
+  alterDefault: true,
+  comment: true,
   createIndex: true,
   dropIndex: true,
   rebuildIndex: true,
   indexType: true,
   indexInclude: true,
   indexFilter: true,
+  indexComment: true,
 });
 
 const oracleCapabilities = capabilities({