fix(desktop): restore custom chat window header

Revert to frameless window with custom header containing
minimize/close buttons and drag region.

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

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

apps/desktop/src-tauri/src/chat.rs

@@ -718,9 +718,9 @@ fn open_chat_window_internal(app: &tauri::AppHandle, ticket_id: &str) -> Result<
     .inner_size(380.0, 520.0)
     .min_inner_size(300.0, 400.0)
     .position(x, y)
-    .decorations(true) // Usar decoracoes nativas do Windows
+    .decorations(false) // Sem decoracoes nativas - usa header customizado
     .always_on_top(true)
-    .skip_taskbar(false) // Mostrar na taskbar
+    .skip_taskbar(true)
     .focused(true)
     .visible(true)
     .build()