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/db/db.ts

@@ -104,6 +104,16 @@ export class TtrpgDatabase extends Dexie {
     // v10 — Phase 2: characters gain `portrait`, map tokens gain `image`
     // (both optional data URLs). No backfill needed.
     this.version(10).stores({});
+
+    // v11 — Phase 3: maps gain walls/doors/lights (UVTT import + dynamic vision).
+    // Backfill empty arrays so code can read them on pre-existing maps.
+    this.version(11).stores({}).upgrade(async (tx) => {
+      await tx.table('maps').toCollection().modify((m: Record<string, unknown>) => {
+        if (m.walls === undefined) m.walls = [];
+        if (m.doors === undefined) m.doors = [];
+        if (m.lights === undefined) m.lights = [];
+      });
+    });
   }
 }