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/features/world/map/MapCanvas.tsx

@@ -36,6 +36,8 @@ export interface CanvasView {
   revealed: string[];
   tokens: CanvasToken[];
   drawings: CanvasDrawing[];
+  /** sight-blocking walls, drawn faintly for the GM (not sent to players) */
+  walls?: { points: Point[] }[] | undefined;
 }
 
 export interface Overlay {
@@ -137,7 +139,7 @@ export function MapCanvas({ view, viewportHeight = '70vh', readOnly, playerFog,
       drawScene(canvasRef.current, imgRef.current, size.w, size.h, {
         vp, W, H, gridSize, cols, rows,
         showGrid: view.showGrid, fogEnabled: view.fogEnabled, revealed: view.revealed,
-        playerFog: !!playerFog, drawings: view.drawings, overlay,
+        playerFog: !!playerFog, drawings: view.drawings, overlay, walls: view.walls,
       });
     });
     return () => cancelAnimationFrame(raf);
@@ -228,7 +230,7 @@ interface SceneParams {
   vp: Viewport;
   W: number; H: number; gridSize: number; cols: number; rows: number;
   showGrid: boolean; fogEnabled: boolean; revealed: string[]; playerFog: boolean;
-  drawings: CanvasDrawing[]; overlay?: Overlay | undefined;
+  drawings: CanvasDrawing[]; overlay?: Overlay | undefined; walls?: { points: Point[] }[] | undefined;
 }
 
 /** Draw the whole scene at device resolution with a camera transform. */
@@ -291,6 +293,21 @@ function drawScene(canvas: HTMLCanvasElement | null, img: HTMLImageElement | nul
   cam();
   for (const d of p.drawings) drawShape(ctx, d);
 
+  // Walls (GM only — faint, so imported line-of-sight is visible while editing)
+  if (p.walls?.length) {
+    cam();
+    ctx.strokeStyle = 'rgba(80,200,255,0.6)';
+    ctx.lineWidth = 2 / zoom;
+    ctx.lineJoin = 'round'; ctx.lineCap = 'round';
+    for (const w of p.walls) {
+      if (w.points.length < 2) continue;
+      ctx.beginPath();
+      ctx.moveTo(w.points[0]!.x, w.points[0]!.y);
+      for (const pt of w.points.slice(1)) ctx.lineTo(pt.x, pt.y);
+      ctx.stroke();
+    }
+  }
+
   // Overlay (AoE/measure/draft/poly/brush)
   const o = p.overlay;
   if (o) {