Action detail pane #28

Problem

The only primary control on an action row is a link to github.com. Needly costs the user a tab switch and saves them nothing, because no information is available before the click to help decide whether the click is worth it.

docs/productidea.md section 15 is right that Needly must never rebuild code review. But owning "everything before the review starts" requires showing enough to triage.

Scope

A split-pane detail panel for the selected action. Read-only: no diff viewer, no comment box, no merge button.

Show:

• Pull request or issue body, truncated with expand
• Labels, milestone, draft state
• Diff size: files changed, additions, deletions
• Check runs with conclusion and a link to the failing job
• Reviewer list with each reviewer's current state
• The last few comments, newest first
• Review risk signals once available

Acceptance criteria

• Selecting a row with Enter or a click opens the pane without navigating away
• Detail data is fetched lazily and cached, never on initial inbox load
• The pane degrades gracefully when the GitHub API is rate limited or the data is incomplete
• The pane is collapsible and its state persists per user
• Layout stays usable below 1024px by switching to a full-width overlay

Out of scope

Diff rendering, inline comments, approving, merging. Those stay on GitHub.