Allow arrow keys to choose terminal-close confirmation actions

[davzucky]

Closing an integrated terminal on Linux with Ctrl+W opens the "Close terminal" confirmation. Left and Right Arrow do not move focus between Cancel and Confirm. The reported experience is that choosing an answer from the keyboard is unclear.

Steps to reproduce:

1. Open and focus an integrated terminal in T3 Code on Linux.
2. Press Ctrl+W to open the close confirmation.
3. Press Right Arrow, then Left Arrow, to try to select an answer.

The requested behavior is for Left Arrow to focus Cancel and Right Arrow to focus Confirm, while keeping Cancel as the initial selection and preserving the existing Tab/Shift+Tab, Enter/Space, and Escape behavior.

This is a focused follow-up to the terminal-dialog portion of #13427. In the maintainer assessment, Tab is identified as the existing navigation method, arrow-key movement is classified as new behavior, and the dialog requests are separated from the launcher shortcut bug. That is why this request is in Ideas.

Investigation against main at 250e052f44dd313b658abebc707242a7b25be340:

• The dialog comes from the shared confirmation host, used by web and desktop.
• A DOM regression test opens the real host from a focused terminal-like textarea. Initial focus lands on Cancel; Right Arrow leaves it there. Escape cancels and returns focus to the textarea.
• A small local implementation adds Left/Right Arrow navigation in that host. The focused dialog, coordinator, terminal-confirmation, and shortcut tests pass: 14 tests across four files. Targeted lint and web typechecking also pass.

The Linux report did not establish that Tab/Shift+Tab fails, and the desktop app was not verified directly. The installed release version was not supplied.

Would this narrow arrow-key addition be acceptable for a focused contribution?