Build the rest of D16 and DT7: named merge failure banners, Queue-row step, and the blocker list with links #199

Problem

Build step 4 is done (#174 via #197, #176 via #195, and the merge job's step list and Cancel merge via #194). Three parts of the design that are tagged to it are not built:

• D16, named failure banners. Each merge-job failure should end in a named banner that jumps to the first affected plan item, for example "Rebase changed P2 and P4. Review them again." or "Checks failed on the rebased code: pnpm test in P3." Today the banner gives the job's reason as text, without a link to the item.
• D16, the Queue row. The task row on the Queue screen should show the merge job's current step. The Queue screen itself is not built yet (build step 7).
• DT7, the blocker list with links. While blockers remain, the merge control reads "N blockers", and its list should let each line jump to its fix (D20). Today the list shows the blocker messages without links.

Scope

Design: docs/designs/codeboost-plan-indexed-review.md, "While merging (D16)", "Merge blockers (D20)" and DT7. Code: web/public/app.js (merge control, banner and blocker dialog) and the merge status the server sends (runner/merge.ts, runner/merge-job.ts), which would need each blocker's and failure's affected item.

Acceptance

• Each failure reason the job can end with names its affected items where it has them, and the banner links to the first one.
• Each blocker in the list links to the item or control that fixes it.
• The Queue-row step lands with the Queue screen (build step 7), or is split into its own issue there.
• Browser tests for each link; DESIGN.md followed.

Found during the independent review of #194 (round 15).