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>