feat(settings): add home content customization and config injection

- Add home_content setting for custom homepage (HTML or iframe URL)
- Inject public settings into index.html to eliminate page flash
- Support ETag caching with automatic invalidation on settings update
- Add Vite plugin for dev mode settings injection
- Refactor HomeView to use appStore instead of local API calls

backend/internal/service/settings_view.go

@@ -31,6 +31,7 @@ type SystemSettings struct {
 	APIBaseURL   string
 	ContactInfo  string
 	DocURL       string
+	HomeContent  string
 
 	DefaultConcurrency int
 	DefaultBalance     float64
@@ -58,6 +59,7 @@ type PublicSettings struct {
 	APIBaseURL          string
 	ContactInfo         string
 	DocURL              string
+	HomeContent         string
 	LinuxDoOAuthEnabled bool
 	Version             string
 }