web: expand a signup row to show its custom fields and metadata

[d4mr]

Summary

The signups table shows email, status, source and date. Anything a waitlist collects through custom fields never appeared in the admin, so you had to export a CSV to read it.

Each row now expands (click the row or the chevron in the new first column) into a details panel:

• Custom fields, in the order the waitlist defines them, with their labels. Empty values show as a dash so the shape is the same on every row. Values submitted under a key that's since been removed from the schema are still listed by key.
• Details: signed up, confirmed, unsubscribed (when set), source, IP address and user agent.

Expanded state is keyed by signup id, so it survives paging and refetches. The row actions menu stops propagation so opening it doesn't toggle the row. Skeleton and empty states account for the extra column.

No API change: the list endpoint already returned the full row including customData.

Test plan

• bun run typecheck
• Local D1 with a waitlist that has text/select/textarea custom fields; rows with full, partial and empty customData expand correctly, multi-line notes keep their line breaks
• Menu on a row opens without toggling the row; chevron and row click both toggle
• Deployed to a production instance (waitlist.veloccy.com) and verified behind Cloudflare Access

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