feat(canvas): 画布工作区 - 整合侧栏与风格预设并修复提示词回填

选择性整合 #547，保留主分支节点与工具条设计。新增默认折叠的节点、资产、任务和历史面板，风格与预设保留在节点内，移除重复入口。

补齐工具权限、提示词解析、资源引用保护及数据库迁移，修复提示词清空和连续编辑后的生成配置同步。

验证：前端全量 1925 项测试、构建、类型检查及 lint 通过；后端 go test ./... 通过。真实模型、PostgreSQL 实库与登录态页面验收仍待进行。

Refs #547

Co-authored-by: bensharp <bensharp@qq.com>

backend/internal/database/migrations.go

@@ -11,7 +11,7 @@ import (
 	"gorm.io/gorm"
 )
 
-const CurrentSchemaVersion int64 = 29
+const CurrentSchemaVersion int64 = 31
 
 const baselineSchemaChecksum = "sha256:open-ai-canvas-schema-v1-20260830"
 const schemaMigrationAppliedAtIndexChecksum = "sha256:schema-migrations-applied-at-index-v2-20260830"
@@ -99,6 +99,12 @@ var schemaMigrations = []migration{
 	{version: 29, name: "agent_resource_leases", checksum: "sha256:agent-resource-leases-v29-20260919", apply: func(tx *gorm.DB) error {
 		return tx.AutoMigrate(&model.CloudAgentResourceLease{})
 	}},
+	{version: 30, name: "builtin_tools", checksum: "sha256:builtin-tools-v30", apply: func(tx *gorm.DB) error {
+		return tx.AutoMigrate(&model.Tool{})
+	}},
+	{version: 31, name: "tool_favorites", checksum: "sha256:tool-favorites-v31", apply: func(tx *gorm.DB) error {
+		return tx.AutoMigrate(&model.ToolFavorite{})
+	}},
 }
 
 func migrateChannelCreditCost(tx *gorm.DB) error {