On a fork, the terminal's forks screen proposes the fork's landed changes to its parent with `u` after a confirm, as the following page's Propose button does #7449

Intent: contributing back to what you forked is one key from the terminal. Today only the following page and its API call proposeUpstream (src/actions/upstream.ts).

Done:

• [ ] On a repository that is a fork, the forks screen (src/tui/screens/forks.ts) offers u to a writer of the fork who is a person. It says how many landed changes would be proposed and to which repository, and only after a confirm calls proposeUpstream.
• [ ] When a proposal is already open (openProposal), the screen says #N is open on owner/name and u opens that pull instead of making another.
• [ ] nothing_to_propose, not_a_fork and refusals are said in the following page's words.
• [ ] A test (e.g. test/tui-forks-propose.test.ts) lands a change on a fork, presses u, confirms, and checks that one pull is open on the parent. A second u opens the same pull, and a reader is not offered u.

Files: src/tui/screens/forks.ts, a new test.