Adds DEFAULT_LAYOUT + IMAGE_FRAME_ROWS constants (fractional art/text window
anchors), a ZoneEditor component for bounding-box layout editing in the
designer, and expands the custom-frames/games CRUD API surface to support
frame image storage and retrieval. Custom card designer pages wire these
together with the existing PNG export pipeline.
See .convoys/card-designer-image-frames.md for scope tracking.
