[P2][Plan 5] SF-025 — Connect rebuilt ZIP export to the product UI

[Ankit6149]

Confirmed problem

A ZIP export API exists, but the Review interface exposes only Markdown and JSON. The feature is effectively unreachable, and its current backend implementation is not yet trustworthy enough to expose directly.

Scope

• Rebuild ZIP generation from the canonical campaign/export projector.
• Add ZIP to a clear Export Center only after channel-format correctness passes.
• Show included channels, generation run, edited channels, warnings, and file layout before download.
• Keep authorization behavior consistent with other exports.

Acceptance criteria

• Users can initiate ZIP export from Review/Export Center.
• ZIP always represents the current authoritative drafts.
• Export is blocked or clearly archival when the campaign is stale.
• The summary matches the actual files inside the archive.
• Failures provide a recoverable error without losing campaign state.

Required verification

• ZIP content snapshot tests for all destination types.
• UI-to-API integration test.
• Stale, edited, partial-failure, and authorization tests.

Dependency

Do not connect the UI until SF-005 and SF-026 are complete.