feat: Add interactive branch switch command

[mark-pro]

Summary

Add a new command dgr switch that provides both direct branch switching and an interactive tree-based branch selection UI.

User Stories

• As a developer, I want to quickly switch between branches in my stack so that I can work on different parts of a feature without typing long branch names.
• As a developer, I want to see a visual tree of my branches when switching so that I understand the context of the branch I am switching to.

Acceptance Criteria

• dgr switch enters interactive mode if no branch is specified.
• Interactive mode displays an indented tree of tracked branches.
• Current selection is marked with ->.
• Arrow keys and hjkl navigate the selection.
• Enter switches to the selected branch and prints a confirmation message.
• Esc or q exits interactive mode without switching.
• dgr switch <branch> switches directly to <branch> if it exists.
• The UI includes a footer listing available shortcuts.

Implementation Notes

• UI Framework: Use ratatui for the interactive TUI.
• Branch Data: Leverage crate::core::tree::run(&TreeOptions::default()) to get the TreeView.
• Tree Rendering: Flatten the TreeView into a list for easier selection management while maintaining indentation levels.
• Git Integration: Use crate::core::git::switch_branch(branch_name) for the actual switch.
• CLI Structure:
  • Add switch to src/cli/mod.rs.
  • Create src/cli/switch/mod.rs for command handling.
  • Create src/cli/switch/interactive.rs for TUI logic.