[Feature] Remove scene-board controls from Replay while preserving multi-board support

[chdenat]

Context

Studio supports multiple widget boards. Replay composition, Draft capture, and HQ
export must use the video board, but Replay-facing UI and lifecycle paths still
carry scene-board assumptions.

Requested behavior

Remove scene-board usage from the Replay workflow while preserving the general
multi-board widget architecture.

Replay configuration, preview, Draft capture, and HQ export must resolve their
widgets exclusively from the Replay video board. Scene boards remain available
for non-Replay Studio features.

Acceptance criteria

• Replay does not read, configure, mount, hide, or capture scene-board widgets.
• Replay uses the video board as its only widget source in preparation, Draft,
  HQ export, cancellation, and completion.
• The general multi-board widget model remains supported.
• Non-Replay scene-board behavior remains unchanged.
• Widget positions, ordering, visibility, resizing, and persistence remain
  isolated by board.
• Existing scene-board widgets are not deleted or migrated merely by entering
  Replay.
• Draft and HQ compose the same ordered video-board widget set for a given
  export snapshot.
• Focused UI, integration, and export regression tests cover board isolation.