Findings tab: fixes from the first walkthrough #386

Context

These came from walking through the Findings tab on Molecule 359 (PR #374) in a scratch copy of the Lab. The copy had forgecomments declared in the Molecule's Formula snapshot and a sample round of 5 findings (high, medium, 2 low, info).

These already work: severity grouping, the auto-fix chip, the include checkboxes, auto-collect on open, the Inbox entry, and Dismiss with reason validation and Undismiss.

Issues

1. Internal reviewer key shown. 0:forgecomments is the heading on the collection card and appears under every finding. Show the plugin name, such as "forgecomments".
2. Status contradicts itself. A failed collection reads "failed … round complete" on one line. Show the failure and its reason only.
3. Finding body missing. Only the title is shown, so the reviewer's explanation is lost. Show the body, collapsed if it's long.
4. Merged PR still offers work. On a merged PR, "Propose a fix round" stays enabled and the Inbox still says "Reviewer findings to triage · 5 open", while the card below says "Nothing to do · merged on GitHub". After merge or close, disable Propose with a reason and drop the Inbox entry.
5. Refusal text is empty. A refused proposal shows "Refused (refused)" with no reason. The CLI had the real one: "This Lab's Formula declares no reviewers". Show that reason in the UI.
6. Selection lost after a refusal. The ticked "Include in the fix round" box is cleared after a refusal. Keep it.
7. Summary says "Reviewer: No review step" next to a declared reviewer. Make the summary row reflect the declared reviewers.
8. Gap in the dismiss form. There's a large gap between the "Why dismiss it?" label and its input.
9. Double auto-collect. Opening the tab collected twice within about a minute. Collect at most once per open, or throttle it.
10. Propose shown with nothing to propose. With no open findings, a disabled "Propose a fix round" still shows. Hide it.

Related mismatch, worth a decision

The tab decides whether to show from the Molecule's own Formula snapshot (findingsSnapshot, lab/internal/ui/findings.go:167). Propose checks the Lab's active approved Formula. If the two differ, the tab shows and Propose is refused. Pick one source of truth, or show the mismatch as the refusal reason (see item 5).

Done when

• Each item is fixed, or split into its own Issue with a reason.
• UI tests cover items 2, 4, 5 and 6.