Add fields and contents to bins; fields to bin types

- bins: fields:{} and contents:[] on all new records
- bin types: fields:{} on all new records
- PUT /api/bins/:id accepts fields
- PUT /api/bin-types/:id accepts fields
- POST/PUT/DELETE /api/bins/:id/contents for content items
  (type: 'component'|'item', component_id or name, quantity, notes)
- api.mjs: add_bin_content, update_bin_content, delete_bin_content

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/lib/api.mjs

@@ -73,6 +73,9 @@ export const create_bin_from_source = (source_id, name) => req('POST', '/api/bin
 export const get_bin         = (id) => req('GET', `/api/bins/${id}`);
 export const update_bin      = (id, body) => req('PUT', `/api/bins/${id}`, body);
 export const update_bin_corners = (id, corners, phys_w, phys_h) => req('PUT', `/api/bins/${id}/corners`, { corners, phys_w, phys_h });
+export const add_bin_content    = (id, body) => req('POST',   `/api/bins/${id}/contents`, body);
+export const update_bin_content = (id, cid, body) => req('PUT', `/api/bins/${id}/contents/${cid}`, body);
+export const delete_bin_content = (id, cid) => req('DELETE',  `/api/bins/${id}/contents/${cid}`);
 export const delete_bin      = (id) => req('DELETE', `/api/bins/${id}`);
 
 export async function upload_bin(file, name) {