feat(ipad): facade canvas authoring mode

[nigel-hall-codes]

Parent: #278 (building-specific facade canvas) — see sdlc/#278/design.md §Coordinate model + §MVP slice.

A mode inside the #276 iPad authoring client (not a separate app):

• Pick a specific building, then a facade (from the bake sidecar's ranked street_facades[] + non-street faces).
• Canvas space = the facade's unit square (x,y ∈ [0,1], resolved to real width/height at import) — the same facade frame templates use, so center-on-iPad lands center-in-Unity.
• Freehand draw (strokes) + place images / request AI signs (feat(server): AI sign generation - provider abstraction #275) as layers; reorder layers.
• Save the layered canvas document to the server (feat(server): canvas storage + flatten-on-export to facadeDecals override #281); the server owns flatten-on-export.

Blocked-by #276 (iPad client design must reserve the canvas slot) and #281 (server canvas endpoints).