Add grid image system with multi-panel support and SPA routing

- Grid images: photograph component boxes in sub-sections, assemble
  into one logical grid via perspective warp (homography)
- Source image gallery: bulk upload photos separately from grid setup
- Grid drafts: persist partial work, resume across sessions
- Multi-panel grids: define rows/cols per photo, system computes panel
  layout; process partially configured grids, edit individual panels
- Pan/zoom canvas editor (HiDPI-aware, touch support) for corner alignment
- SPA routing with canonical URLs (history.pushState, server catch-all)
- Express error visibility: uncaughtException/unhandledRejection handlers
  and 4-arg error middleware
- Original filename stored on source image upload
- Various null-safety fixes and CSS [hidden] override fixes

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

public/lib/api.mjs

@@ -27,3 +27,19 @@ export const get_inventory      = () => req('GET', '/api/inventory');
 export const create_inventory   = (body) => req('POST', '/api/inventory', body);
 export const update_inventory   = (id, body) => req('PUT', `/api/inventory/${id}`, body);
 export const delete_inventory   = (id) => req('DELETE', `/api/inventory/${id}`);
+
+// Grid drafts
+export const get_grid_drafts    = () => req('GET', '/api/grid-drafts');
+export const create_grid_draft  = (body) => req('POST', '/api/grid-drafts', body);
+export const update_grid_draft  = (id, body) => req('PUT', `/api/grid-drafts/${id}`, body);
+export const delete_grid_draft  = (id) => req('DELETE', `/api/grid-drafts/${id}`);
+
+// Source images
+export const get_source_images   = () => req('GET', '/api/source-images');
+export const delete_source_image = (id) => req('DELETE', `/api/source-images/${id}`);
+
+// Grid images
+export const get_grids        = () => req('GET', '/api/grid-images');
+export const get_grid         = (id) => req('GET', `/api/grid-images/${id}`);
+export const delete_grid      = (id) => req('DELETE', `/api/grid-images/${id}`);
+export const update_grid_panel = (id, pi, body) => req('PUT', `/api/grid-images/${id}/panels/${pi}`, body);