fix(web): pass onMerge to AttentionZone in kanban view

[0xGermain]

Summary

• Pass onMerge={handleMerge} to AttentionZone in the kanban layout — the prop was missing, making the merge button a silent no-op
• Parse JSON error responses from /api/prs/:id/merge so the toast shows human-readable blocker reasons instead of raw JSON

What happened

The handleMerge callback was wired to BottomSheet (mobile preview) but never forwarded to AttentionZone in the desktop kanban grid. Since SessionCard declares onMerge as optional (onMerge?.()), clicking the merge button silently did nothing — no fetch, no error, no toast.

Additionally, when a merge does fail (e.g. CI blockers), the error toast dumped raw JSON like {"error":"PR is not mergeable","blockers":["CI is failing"]}. Now it shows Merge blocked: CI is failing.

Changes

packages/web/src/components/Dashboard.tsx

1. Added onMerge={handleMerge} to the AttentionZone component in the kanban map() (line ~723)
2. Parse the merge API error response JSON to extract blockers or error for the toast message

How to test

1. Start AO with a project that has an open PR in the "merge" kanban column
2. Click the "Merge PR #N" button on the kanban card
3. Before fix: nothing happens, no network request in DevTools
4. After fix: merge request fires; on success a "PR merged" toast appears; on failure a readable error toast appears (e.g. "Merge blocked: CI is failing")

Closes #TBD