fix(ui): remove Tests tab, remove raw-md/collapse toggles, add Copy MD button, bump v0.9.1

- Remove Tests tab from navigation (browsertest still works via snippet/extension)
- Remove showRawMarkdown and collapseHistory toggles from Studio input bar
- Add "Copy MD" button on each assistant message header to copy raw markdown
- Bump version to 0.9.1

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

internal/version/version.go

@@ -7,7 +7,7 @@ import (
 
 const (
 	Name    = "muyue"
-	Version = "0.9.0"
+	Version = "0.9.1"
 	Author  = "La Légion de Muyue"
 )