Adds the actual "Android home screen" placement experience: a new Layout tab with a pointer-driven canvas for dragging/resizing widgets and adding new ones from a type picker. Backed by a new routers/api_widgets.py (create/move/delete), which re-validates bounds, minimum footprint, and no-overlap server-side regardless of what the client already checked. A widget added without an explicit position lands in the first open space that fits it (grid.find_open_rect), so users don't have to hunt for empty space themselves. Also fixes a real latent bug this surfaced: changing a frame's orientation swaps the widget grid's long/short axis, which left existing widget placements out of bounds on the new grid with no render-time safeguard. Orientation changes now reset the layout to a single full-panel widget (keeping the first widget's type, dropping the rest), with a client-side confirm before it happens.