fix(ui): add missing copiedMsg state, remove dead code, bump v0.9.3

The copiedMsg state was referenced in the Copy MD button but never declared,
causing a ReferenceError crash at runtime. Also removed unused compress
logic (collapseHistory/forceExpand), dead functions (renderContent, formatText,
CodeBlockWithCopy, MermaidBlock), and unused imports (useMemo, mermaid).

🤗 Generated with Crush

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

internal/version/version.go

@@ -7,7 +7,7 @@ import (
 
 const (
 	Name    = "muyue"
-	Version = "0.9.2"
+	Version = "0.9.3"
 	Author  = "La Légion de Muyue"
 )