RoomsPanel drag-drop: no rollback on partial upload failure

[PythonFZ]

Context

In `frontend/src/panels/RoomsPanel.tsx:62-69`, the drag-drop flow performs two dependent operations:

1. Create a new room (`POST /v1/rooms`).
2. Upload the dropped file into that room.

If step 2 fails, the room created in step 1 stays behind as an orphan — no rollback, no retry affordance. User ends up with an empty room they did not intend to create, and the file they actually wanted to upload is not attached anywhere.

Repro

1. Drag a file onto RoomsPanel while the server is reachable for `POST /v1/rooms` but the upload endpoint times out / is rejected (e.g. file too large, network blip mid-transfer, auth drops between the two requests).
2. Observe: new empty room appears in the list, file never lands.

Acceptance

Pick one:

• Compensating delete — on upload failure, `DELETE` the just-created room and surface the original upload error.
• Retry / discard UX — keep the room on the client, show an inline error with "Retry upload" and "Discard room" actions.
• Server-side atomic endpoint — introduce `POST /v1/rooms?seed=` so creation and seeding are a single request (probably the cleanest long-term answer).

No preference; whichever fits the broader room-creation UX story.

Related

Flagged during PR #920 review (Minor, deferred).