feat(ui): add read-only interactive issue browser

[camopy]

Summary

Add a read-only docket ui that gives docket a keyboard-first terminal browser with list, board, and synced issue detail views.

Why

docket is effective for direct commands and automation, but browsing issues as a human still takes multiple commands. This PR introduces a lightweight interactive TUI so people can inspect work quickly without changing the existing CLI model.
This PR is also meant as a direction check before expanding the full read-only TUI roadmap.

Related: #7

What Changed

• added docket ui
• added default list view
• added board view
• added synced issue detail pane
• extracted shared read loaders into internal/app
• reused those loaders from existing CLI read commands
• fixed a board-view terminal input freeze seen under cmux/ghostty
• added targeted regression tests for the freeze path

Scope

This PR is intentionally read-only.
Included:

• browse issues in list
• browse issues in board
• inspect selected issue details
• refresh and keyboard navigation
  Not included:
• create/edit/move/comment
• search/filter UI
• vote views
• next, plan, stats, config, version
• graph/log/comments/files dedicated screens

Testing

Ran:

• go test ./internal/tui ./internal/app ./internal/cli
• targeted regression tests for board freeze behavior
  Notes:
• regression tests now cover probe-key filtering, board-to-list switching, help after board load, and stale detail message handling