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>

e2e/fixtures/sample.dd2vtt

@@ -0,0 +1,8 @@
+{
+  "format": 0.3,
+  "resolution": { "map_origin": { "x": 0, "y": 0 }, "map_size": { "x": 4, "y": 3 }, "pixels_per_grid": 100 },
+  "line_of_sight": [[{ "x": 1, "y": 1 }, { "x": 3, "y": 1 }, { "x": 3, "y": 2 }]],
+  "portals": [{ "position": { "x": 3, "y": 1.5 }, "bounds": [{ "x": 3, "y": 1 }, { "x": 3, "y": 2 }], "closed": true }],
+  "lights": [{ "position": { "x": 2, "y": 2 }, "range": 4, "color": "ffd9a0", "intensity": 0.6 }],
+  "image": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg=="
+}

e2e/maps.spec.ts

@@ -38,6 +38,21 @@ test('maps: upload, place token, reveal all, show to players', async ({ page })
   await expect(page.getByTestId('map-token')).toHaveCount(1);
 });
 
+test('maps: import a Universal VTT (.dd2vtt) map', async ({ page }) => {
+  await page.getByRole('button', { name: '+ New campaign' }).first().click();
+  await page.locator('input[data-autofocus]').fill('VTTimport');
+  await page.getByRole('button', { name: 'Create' }).click();
+  await page.getByRole('link', { name: 'Dashboard' }).click();
+  await page.getByRole('link', { name: 'Maps' }).click();
+
+  await page.locator('input[type="file"]').setInputFiles('e2e/fixtures/sample.dd2vtt');
+
+  // Imported map opens in the editor with the file's pixels_per_grid as the grid size.
+  await expect(page.getByTestId('map-viewport')).toBeVisible();
+  await expect(page.getByLabel('Grid cell size')).toHaveValue('100');
+  await expect(page.getByRole('button', { name: 'Export .uvtt' })).toBeVisible();
+});
+
 test('maps: zoom controls scale the canvas and fit resets', async ({ page }) => {
   await page.getByRole('button', { name: '+ New campaign' }).first().click();
   await page.locator('input[data-autofocus]').fill('Zoom');