Broaden TPEN_CONTEXT.siblings to span all layers, not just the active one

[thehabes]

Surfaced during testing of the 5-2-tools-align cut (PR #564). The TPEN_CONTEXT.canvases array sent to tool iframes only includes pages from the single layer that contains the active page. For a project with multiple layers (e.g. "Layer 1 Page 1" + "Layer 2 Page 1"), only the active layer's pages reach the tool — Compare-Pages' dropdown is correspondingly limited.

This is a pre-existing gap, not introduced by #564. The legacy CANVASES payload (now removed) used the same single-layer .find(layer)?.pages?.flatMap(...) shape (see 1196c28:components/simple-transcription/index.js lines 1070-1072), so #getActiveLayerCanvases was a faithful 1:1 port — but the constraint pre-dates the contract cleanup and is worth lifting.

Required change

Broaden #getActiveLayerCanvases (in components/simple-transcription/index.js) to flatten across all layers, not just the active one. Likely shape:

#getProjectCanvases() {
  return TPEN.activeProject?.layers
    ?.flatMap(l => l.pages ?? [])
    .map(p => ({ id: p.target, label: p.label })) ?? []
}

Consider renaming to #getProjectCanvases since "active layer" no longer describes the result. Update the JSDoc on #buildTPENContext to clarify the list spans all layers.

Companion repo

Compare-Pages (CenterForDigitalHumanities/Compare-Pages) consumes canvases directly without filtering, so no code change is needed there once this lands. Its PR CenterForDigitalHumanities/Compare-Pages#3 will start showing the cross-layer list automatically.

Verification

On a multi-layer project, open /transcribe, attach Compare-Pages, and confirm the dropdown lists every page across every layer.