Implement custom window decorations (CSD) for the Desktop (JVM) target

[MessiasLima]

Description

Depends on: Create :sample:kombu Gradle module with all KMP targets, per-platform windowing, and :umami-api dependency.

Replace the default OS window decorations on Desktop with Compose-drawn custom chrome that matches the Kombu theme.

• Scope and constraints
  • Use Compose for Desktop's window decoration API to draw a custom title bar.
  • The title bar must include minimize, maximize, and close buttons.
  • Window remains draggable and resizable.
  • Dark / light theme variants should match the rest of the app.

Scope

• In scope: Desktop window chrome, title bar buttons, theme-aware colors.
• Out of scope: theme definition (handled in the theme ticket), other platforms.

Acceptance criteria

• Desktop window uses custom decorations on macOS, Linux, and Windows.
• Minimize / maximize / close buttons work.
• Window can still be dragged and resized.
• Title bar colors update when theme changes.

Additional information

Compose Desktop supports WindowDecoration on Window for client-side decorations.