feat✨: add sys_columns.col_width and default_value (PRD 010 F1/F2)

Back the code generator's Vue 3 template migration: col_width lets R2's
column-width inference be overridden per field, and default_value lets
R1/A6's "unconfigured rows still generate a usable page" guarantee hold
for generated forms. Both use a sentinel default (0 / "") rather than
NULL so "unconfigured" has exactly one representation - see
docs-prd/010-代码生成器前端模板迁移Vue3/数据库变更.md §1.1.

The migration and the model change land together: AddColumn reads the
column definition off tools.SysColumns's own gorm tag, so splitting them
across commits would leave one of them failing to compile.

app/other/models/tools/sys_columns.go

@@ -45,6 +45,19 @@ type SysColumns struct {
 	CreateBy           int          `gorm:"column:create_by;size:20;" json:"createBy"`
 	UpdateBy           int          `gorm:"column:update_By;size:20;" json:"updateBy"`
 
+	// ColWidth and DefaultValue back PRD 010 F1/F2 (代码生成器前端模板迁移 Vue 3).
+	// Both use a sentinel default (0 / "") rather than NULL - see
+	// docs-prd/010-代码生成器前端模板迁移Vue3/数据库变更.md §1.1: a non-pointer
+	// int/string field can never read NULL back out, and NULL would give
+	// "unconfigured" two representations instead of one. Callers test
+	// ColWidth == 0 / DefaultValue == "" to detect "not configured".
+	//
+	// ColWidth deliberately has no gorm size tag: this codebase's "size:N"
+	// convention on numeric fields maps to a narrow SQL integer type (see
+	// column_width_test.go), and col_width needs to hold values up to 800.
+	ColWidth     int    `gorm:"column:col_width;not null;default:0;comment:table column width in px, 0 = not configured" json:"colWidth"`
+	DefaultValue string `gorm:"column:default_value;size:255;not null;default:'';comment:form field default value, empty = not configured" json:"defaultValue"`
+
 	common.ModelTime
 }
 

cmd/migrate/migration/version/1786700010000_gen_column_layout_fields.go

@@ -0,0 +1,56 @@
+package version
+
+import (
+	"runtime"
+
+	"gorm.io/gorm"
+
+	"go-admin/app/other/models/tools"
+	"go-admin/cmd/migrate/migration"
+	common "go-admin/common/models"
+)
+
+// Add sys_columns.col_width and sys_columns.default_value for PRD 010 F1/F2
+// (代码生成器前端模板迁移 Vue 3).
+//
+// col_width backs R2's column-width inference fallback and default_value
+// backs R1/A6's "unconfigured rows still generate a usable page" guarantee -
+// see docs-prd/010-代码生成器前端模板迁移Vue3/数据库变更.md §1.1 for why both
+// defaults are sentinels (0 / "") rather than NULL: a non-pointer Go int/
+// string field can never read NULL back out, and NULL would give
+// "unconfigured" two representations instead of one.
+//
+// Ordered after 1786700003000, so this reads tools.SysColumns (the runtime
+// model sys_columns's Update/GetPage/GetSysTablesInfo actually query through)
+// rather than cmd/migrate/migration/models, matching every migration in this
+// directory since sys_columns was converted - see
+// 1786700004000_generator_tables_marker.go and schema_coverage_test.go's
+// TestPostConversionMigrationsAvoidFrozenSeedModels.
+//
+// Hard prerequisite: tools.SysColumns must already declare ColWidth and
+// DefaultValue (with the gorm tags in the doc above) by the time this file
+// is compiled - AddColumn reads the column definition off the struct's own
+// tag, not off anything in this file. Landing this migration without that
+// model change first makes HasColumn/AddColumn silently do nothing (the
+// field lookup fails and AddColumn returns an error naming the missing
+// field), which fails loudly rather than silently - see the "no such field"
+// error - so this is caught at migrate time, not left for a report later.
+func init() {
+	_, fileName, _, _ := runtime.Caller(0)
+	migration.Migrate.SetVersion(migration.GetFilename(fileName), _1786700010000GenColumnLayoutFields)
+}
+
+func _1786700010000GenColumnLayoutFields(db *gorm.DB, version string) error {
+	m := db.Migrator()
+	if !m.HasColumn(&tools.SysColumns{}, "ColWidth") {
+		if err := m.AddColumn(&tools.SysColumns{}, "ColWidth"); err != nil {
+			return err
+		}
+	}
+	if !m.HasColumn(&tools.SysColumns{}, "DefaultValue") {
+		if err := m.AddColumn(&tools.SysColumns{}, "DefaultValue"); err != nil {
+			return err
+		}
+	}
+	return db.Create(&common.Migration{Version: version}).Error
+}