Add first TUI shell

[tnez]

Context
Once typed dashboard state exists, add the terminal shell with ratatui and crossterm. See docs/v1-scope.md.

Acceptance criteria

• App opens in a terminal.
• App can quit cleanly from the keyboard.
• App renders fake dashboard state before real commands are wired through.
• Focused panel is visually distinct.
• Terminal I/O remains at the edge of the program.

Out of scope

• Final visual polish.
• Live command-backed panels.
• Background refresh.

[tnez]

Implementation is complete in PR #11: #11. I could not close this issue from here because merging PR #11 into main was blocked pending explicit approval for the default-branch mutation. The PR is green and links this issue for automatic closure on merge.