[Widgets] Implement Rust Widget APIs for Calendar, Kanban, and Hierarchical Tree Components

[rorychatt]

Problem Statement

Rusty-Framework frontend includes visual components for Calendar, Kanban boards, and Trees, but the Rust backend (rusty crate) lacks Rust struct definitions and builders.

Missing Rust Widgets

• Calendar & CalendarEvent
• Kanban & KanbanCard
• Tree & TreeNode

Proposed Architecture & Design

1. Create calendar.rs, kanban.rs, and tree.rs in rusty/src/widgets/.
2. Implement drag-and-drop callbacks, item selection states, and event listeners.
3. Re-export all structs in rusty::widgets.

Acceptance Criteria

• Calendar, CalendarEvent, Kanban, KanbanCard, Tree, TreeNode APIs implemented in Rust.
• Serialized virtual tree output validated.
• Re-exported in rusty::widgets.