fix(e2e): pin Web Share off in the desktop export tests (macOS WebKit)

The three "plain desktop browser" export tests (the PNG and GCG downloads plus the
legacy-Telegram clipboard copy) assumed navigator.share is absent — true for Chromium
and for WebKit on the Linux CI host, but desktop WebKit on macOS exposes a working Web
Share API. There shareUrlAsFile / pickGcgDelivery take the share branch, so no download
event fires and the "GCG copied to the clipboard" toast never shows, and the tests
failed only on macOS WebKit.

Pin navigator.share/canShare off in those three tests (a withoutWebShare helper that
mirrors the inverse stub the share-sheet test already uses), so the delivery path under
test is deterministic and identical across engines and OSes. Test-only; no production
change. Full e2e suite: 228 passed on Chromium + WebKit.

ui/e2e/export.spec.ts

@@ -38,7 +38,20 @@ async function openFinishedHistory(page: Page): Promise<void> {
   await expect(page.getByRole('button', { name: 'Export game' })).toBeVisible();
 }
 
+// Desktop WebKit (Safari's engine) exposes a working Web Share API, unlike Chromium and WebKit on
+// Linux (the CI host). The "plain desktop browser" delivery paths tested below are reached only when
+// Web Share is ABSENT — shareUrlAsFile / pickGcgDelivery fall through to the anchor download or the
+// clipboard copy — so pin that precondition; otherwise macOS WebKit takes the share branch and no
+// download or clipboard copy occurs. Mirrors the inverse stub the share-sheet test sets up.
+async function withoutWebShare(page: Page): Promise<void> {
+  await page.addInitScript(() => {
+    Object.defineProperty(navigator, 'share', { value: undefined, configurable: true });
+    Object.defineProperty(navigator, 'canShare', { value: undefined, configurable: true });
+  });
+}
+
 test('the chooser downloads the PNG through the signed URL on a plain browser', async ({ page }) => {
+  await withoutWebShare(page);
   await routeDl(page);
   await openFinishedHistory(page);
   await page.getByRole('button', { name: 'Export game' }).click();
@@ -49,6 +62,7 @@ test('the chooser downloads the PNG through the signed URL on a plain browser',
 });
 
 test('the chooser downloads the GCG through the same signed-URL route', async ({ page }) => {
+  await withoutWebShare(page);
   await routeDl(page);
   await openFinishedHistory(page);
   await page.getByRole('button', { name: 'Export game' }).click();
@@ -198,6 +212,7 @@ test('Telegram on iOS keeps the app chooser and opens the OS share sheet', async
 });
 
 test('a legacy Telegram client (no downloadFile) hides the image and copies the GCG', async ({ page }) => {
+  await withoutWebShare(page);
   await page.addInitScript(() => {
     // Headless engines deny the real clipboard; a permissive stub keeps the legacy
     // copy path deterministic in both browsers.