feat(ipad): building browser + facade picker — list/filter by neighborhood & type, pick building + facade

[nigel-hall-codes]

Implement the Building Browser — the entry point to all authoring actions in the iPad client.\n\n- Sidebar list of buildings, filterable by neighborhood and type (from GET /buildings)\n- Tap a building to see its facts (classification, dims, street_facades[])\n- Facade picker using ranked street_facades[] + derivable Back/Left/Right/Front faces\n- Selecting a building + facade opens the existing FacadeCanvasView\n\nBlocked-by #299 (building facts endpoint). Discovered from #298.