feat: add standalone level editor for visual map editing

Full-featured editor at /editor.html with:
- Visual wall painting (click/drag to paint/erase collision tiles)
- Entity placement: NPCs, exits, interactions, spawn point
- Properties panel for editing dialog, facing, target maps, etc.
- Zoom/pan with scroll wheel and right-click drag
- Tile coordinate overlay on the map PNG backgrounds
- Color-coded overlays matching the F3 debug view
- Export as JSON or as complete maps.js source code
- Import JSON to load/restore map data
- Keyboard shortcuts: 1-7 for tools, Delete to remove entities
- All 4 maps supported: lab, pallet-town, house, route-1

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

Dockerfile

@@ -1,5 +1,6 @@
 FROM nginx:alpine
 COPY index.html /usr/share/nginx/html/
+COPY editor.html /usr/share/nginx/html/
 COPY css/ /usr/share/nginx/html/css/
 COPY js/ /usr/share/nginx/html/js/
 COPY assets/ /usr/share/nginx/html/assets/