Phase 3: Universal VTT (.dd2vtt/.uvtt) import + export

- src/lib/vtt/uvtt.ts: pure, tested parser converting UVTT grid-unit geometry to
  image pixels (pixels_per_grid → gridSize), extracting the base64 image, walls
  (line_of_sight), doors (portals) and lights; plus toUvtt() for export.
- BattleMap gains walls/doors/lights (Dexie v11, backfilled). mapsRepo.createFromVtt.
- MapsPage "+ Add map" now accepts .dd2vtt/.uvtt/.df2vtt (text) and images, and a
  per-map "Export .uvtt" (image dims → map_size). downloadText() helper.
- Walls render faintly for the GM (CanvasView.walls / drawScene) so imported
  line-of-sight is visible; Phase 4 will turn walls into dynamic vision.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/img/resize.ts

@@ -16,6 +16,11 @@ function loadImage(src: string): Promise<HTMLImageElement> {
   });
 }
 
+/** Natural pixel size of an image data URL. */
+export function imageSize(src: string): Promise<{ w: number; h: number }> {
+  return loadImage(src).then((img) => ({ w: img.naturalWidth, h: img.naturalHeight }));
+}
+
 /** Read a File as a data URL. */
 export function fileToDataUrl(file: File): Promise<string> {
   return new Promise((resolve, reject) => {