feat: 品牌重命名 Sub2API -> TianShuAPI

- 前端: 所有界面显示、i18n 文本、组件中的品牌名称
- 后端: 服务层、设置默认值、邮件模板、安装向导
- 数据库: 迁移脚本注释
- 保持功能完全一致，仅更改品牌名称

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/service/geminicli_codeassist.go

@@ -1,13 +1,13 @@
-package service
-
-import (
-	"context"
-
-	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
-)
-
-// GeminiCliCodeAssistClient calls GeminiCli internal Code Assist endpoints.
-type GeminiCliCodeAssistClient interface {
-	LoadCodeAssist(ctx context.Context, accessToken, proxyURL string, req *geminicli.LoadCodeAssistRequest) (*geminicli.LoadCodeAssistResponse, error)
-	OnboardUser(ctx context.Context, accessToken, proxyURL string, req *geminicli.OnboardUserRequest) (*geminicli.OnboardUserResponse, error)
-}
+package service
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+)
+
+// GeminiCliCodeAssistClient calls GeminiCli internal Code Assist endpoints.
+type GeminiCliCodeAssistClient interface {
+	LoadCodeAssist(ctx context.Context, accessToken, proxyURL string, req *geminicli.LoadCodeAssistRequest) (*geminicli.LoadCodeAssistResponse, error)
+	OnboardUser(ctx context.Context, accessToken, proxyURL string, req *geminicli.OnboardUserRequest) (*geminicli.OnboardUserResponse, error)
+}