The terminal's board screen removes the marked panel with `x` after `remove <title>? y/n`, as the web's Remove does #7990

Intent: an admin tidies a board from the terminal without opening the web; today the board screen in src/tui/screens/boards.ts marks a panel with [/] and can pin it (p) but cannot remove it, while the web's Remove posts { kind: "remove", index } through changeBoard.

Done looks like:

• [ ] On a board with panels, a forge admin sees x remove panel in the key line; anyone else does not (the gate is changeBoard's admin, read once like the list's).
• [ ] x puts remove <panel title>? y/n on the head line, as the board list's D does with delete <title>? y/n; n or esc changes nothing.
• [ ] y calls changeBoard(f.app, f.user, slug, { kind: "remove", index: at }) from src/actions/boards.ts, re-reads the board, keeps the mark on a panel that still exists, and toasts "panel removed" or boardRefusal(error).
• [ ] A new test/tui-board-panel-remove.test.ts (driven as test/tui-boards.test.ts drives the screen) proves: x,y leaves the board one panel shorter with the right one gone; x,n changes nothing; a non-admin is not offered x. It fails without the change.
• [ ] bun test test/tui-board-panel-remove.test.ts test/tui-boards.test.ts passes.

Files: src/tui/screens/boards.ts, test/tui-board-panel-remove.test.ts Risks: a pinned panel that is removed: leave pins as changeBoard leaves them on the web, and say so in the pull. Brief: a builder who knows the terminal screens.