[P1][Plan 2] SF-008 — Add persistent failed-channel recovery and retry

[Ankit6149]

Confirmed problem

When a destination fails, the UI still renders its channel tab and a blank editor. The failure reason is mostly hidden in a temporary toast or collapsed notes, and there is no direct retry action.

Scope

• Persist per-channel statuses: queued, generating, revising, complete, needs_review, failed, and cancelled.
• Show the status directly on each channel tab.
• Replace blank failed editors with a recovery panel containing safe error details and Retry destination.
• Preserve all successful and edited channels during retry.
• Store retry count and latest failure class.

Acceptance criteria

• Failed channels never look like empty successful drafts.
• The failure state remains visible after the toast disappears and after save/reopen.
• Retry affects only the selected destination.
• Successful/edited destinations remain unchanged.
• Cancelled and failed states are visually and semantically distinct.

Required verification

• Partial campaign fixture with success, needs_review, failed, and cancelled channels.
• Retry-one-channel integration test.
• Save/reopen status persistence test.
• Keyboard and screen-reader verification for status and recovery controls.

[Ankit6149]

Closing as completed against the issue's original acceptance criteria.

Verified merged evidence:

• PR Make failed destination recovery explicit #325: failed destinations render explicit recovery state and isolated "Retry destination"
• PR Stop active and queued generation on cancellation #326: cancellation stops queued/active work; cancelled is distinct from failed and remains independently retryable
• PR Stream truthful per-destination generation progress #329: live per-destination queued/generating/revising/complete/needs_review/failed/cancelled progress and accessible live status
• PR Persist destination recovery state across save and reopen #331: safe per-destination recovery state persists across save/reopen, including failed destinations with no fabricated draft body
• channel tabs expose status in their accessible name; failed/cancelled recovery surfaces use alert semantics
• CHANNEL regeneration targets exactly the active destination, preserving successful/edited destinations

The remaining durable in-flight resume/cross-process job work is a separate broader execution concern and is not required by #29's stated acceptance criteria.