Session upload now carries a deferred hero-image input; the submit handler
creates the release then POSTs the held hero to the existing resource-addressed
endpoint. Hero is optional with a non-blocking warn-if-missing gate. The
per-row hero upload in CmsSessionBrowser remains the replace/correct path.
Covers AlbumHeaderFields MudTextField label + RequiredError, and the matching
code-side validation messages in BatchEdit and BatchUpload for consistency.
Renames Genre tab to Release Archive with switch-free medium card group
(Enum.GetValues-driven). Adds MediumFields single dispatch + CutFields/SessionFields/
MixFields per-medium sections embedded by all five upload/edit forms. BatchUpload
enforces single-track invariant for Session/Mix. Adds CmsSessionBrowser (hero-image
upload) and CmsMixBrowser (waveform status + per-row Generate trigger).
ICmsReleaseService/CmsReleaseService wraps api/release endpoints.
Note: medium selector is forward-compat only — API write path pending.
daniel-c-harvey