MistDemo: zone-aware writes (CRUD + assets)

[leogdion]

Summary

Extend MistDemo web create/update/delete (and asset upload) to honor a zone selection, building on ZoneSelector from #438 / #453.

Library conveniences already accept zoneID: ZoneID? = nil on createRecord / updateRecord / deleteRecord — the remaining gap is the demo request/backend/browser stack.

Scope

1. Reuse ZoneSelector on WebRequests.Create / Update / Delete (flat zoneName/zoneOwner wire format, same decode rules as query).
2. Thread through WebBackend / CloudKitService+WebBackend / mocks / WebServer+CRUD.
3. Assets — UploadAsset must carry the same zone; create-in-custom-zone after a default-zone upload fails at CloudKit.
4. Browser UX — prefer reusing the query toolbar zone for writes (label clearly) over a second set of inputs; CloudKit JS writes use per-record record.zoneID (saveRecords / deleteRecords), which is a different shape from query.

Out of scope

• Live shared-zone round-trip validation (still open from Zone payloads: deleted, zoneType unmodeled and ownerRecordName never decodes (confirmed live) #444 evidence caveats).
• Broader “Improve Web Interface” blob (Improve Web Interface #390 closed as not actionable).

Depends on

• MistDemo web UI: add zoneName/zoneOwner inputs to query panel #438 / MistDemo web: zoneName/zoneOwner on query panel (#438) #453 (ZoneSelector + query panel zone inputs)
• Model zone payload fields: ownerRecordName, zoneType, deleted (#444) #451 zone owner wire key (ownerRecordName) already on v1.0.0-beta.5