An admin renames or recolours a label from the terminal's Labels screen with `e` #7916

Intent: the terminal's Labels screen does what Settings › Labels and MCP repo_labels update already do, so a label is fixed without leaving the terminal.

Done:

• [ ] on src/tui/screens/labels.ts, e on the picked label opens a prompt prefilled with its name, then its colour; Enter saves through updateLabel in src/actions/repos.ts, Esc leaves it as it was
• [ ] the key is offered (and in the key bar) only to a repository admin; for anyone else e does nothing
• [ ] an invalid colour or a name already taken shows the same words the web page shows (from the actions' words) and keeps the prompt open
• [ ] the list re-reads after a save and keeps the picked row
• [ ] tests: an admin's rename shows in the list; a non-admin's e changes nothing

Waits for #7843 (and #7842), which change the same screen. Files: src/tui/screens/labels.ts, its test.

duplicate of #7868