feat(ipad): part authoring — import screenshot, trace facade elements, produce GLB + assign material roles

[nigel-hall-codes]

Implement Part Authoring mode inside the iPad client (D5: roles only, no colour picker).\n\n- Import a reference screenshot (stays on-device per D6)\n- Trace a facade element outline\n- Assign material roles per submesh (role picker, never a colour picker — #266 D5)\n- Set anchor + mountDepth_m\n- POST /parts (PartDef JSON) then PUT /parts/{id}/glb (the traced GLB)\n- Preview existing parts via GET /parts/{id}/glb (G2, #300)\n\nBlocked-by #300 (asset GET) and #301 (building browser / navigation). Discovered from #298.