fix(terminal): resolve PTY shell exec error, simplify CLI, unify Config tabs, restore Studio CSS

- Fix detectShell() to return full paths via LookPath (was returning bare
  names causing exec error on some systems)
- Add shell path validation before pty.Start to prevent crashes
- Simplify CLI: remove all subcommands, keep only desktop launch with --port
- Restore missing Studio shared CSS (code blocks, input area, animations)
- Replace Config vertical sidebar with horizontal nav-tabs matching main layout

💘 Generated with Crush

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

internal/api/terminal.go

@@ -82,10 +82,19 @@ func (s *Server) handleTerminalWS(w http.ResponseWriter, r *http.Request) {
 		shell := initMsg.Data
 		if shell == "" {
 			shell = detectShell()
+		} else {
+			if path, err := exec.LookPath(shell); err == nil {
+				shell = path
+			}
+		}
+
+		if _, err := os.Stat(shell); err != nil {
+			conn.WriteJSON(wsMessage{Type: "error", Data: fmt.Sprintf("shell not found: %s", shell)})
+			return
 		}
 
 		if strings.Contains(shell, "wsl") {
-			cmd = exec.Command("wsl", "--shell-type", "login")
+			cmd = exec.Command(shell, "--shell-type", "login")
 		} else if strings.Contains(shell, "powershell") || strings.Contains(shell, "pwsh") {
 			cmd = exec.Command(shell, "-NoLogo", "-NoProfile")
 		} else {
@@ -250,8 +259,8 @@ func (s *Server) handleTerminalSessionsDelete(w http.ResponseWriter, r *http.Req
 func detectShell() string {
 	shells := []string{"zsh", "bash", "fish", "pwsh", "powershell"}
 	for _, s := range shells {
-		if _, err := exec.LookPath(s); err == nil {
-			return s
+		if path, err := exec.LookPath(s); err == nil {
+			return path
 		}
 	}
 	return "/bin/sh"