Add interactive TUI mode for git-stack status

Implement fullscreen TUI for branch navigation using ratatui/crossterm.
Users can now run `git stack status -i` to interactively browse the
branch tree and checkout branches with Enter.

Key changes:
- Extract shared rendering infrastructure into src/render/ module
  - colors.rs: ThemeColor type with unified color constants
  - tree_data.rs: RenderableBranch/RenderableTree for flattened tree data
  - cli.rs: CLI rendering using the shared data types
- Add TUI implementation in src/tui/ module
  - input.rs: Keyboard handling (j/k, arrows, Enter, q/Esc)
  - app.rs: App state, cursor navigation, ratatui rendering
- Add -i/--interactive flag to status command
- Route to TUI or CLI based on flag, checkout branch on selection

Navigation: j/↓ down, k/↑ up, Enter checkout, q/Esc quit

Author: wbbradley

Cargo.lock

@@ -15,10 +15,12 @@ anyhow = { version = "1.0.98", features = ["backtrace"] }
 chrono = "0.4.41"
 clap = { version = "4.5.37", features = ["derive"] }
 colored = "3.0.0"
+crossterm = "0.29.0"
 git2 = "0.20"
 indicatif = "0.18.3"
 md5 = "0.8.0"
 rand = "0.9.2"
+ratatui = "0.30.0"
 serde = { version = "1.0.219", features = ["derive"] }
 serde_json = "1.0"
 serde_yaml = "0.9.34"