test(e2e): fix load-induced mobile flakes (gesture timing + hydration)

Three distinct load-induced flakes in the mobile suite, all root-caused from real traces:

1. gestures-longpress "quick tap": longPress() drove page.mouse.down + Node-side
   waitForTimeout(200) + page.mouse.up — three CDP round-trips. Under load the browser saw
   the pointerup >500ms after pointerdown, firing the app's real long-press so the
   ContextSheet opened on a quick tap. New quickTap() helper dispatches pointerdown/up
   entirely in-browser on one clock, so the earlier-due pointerup always cancels the 500ms
   timer first — immune to CDP latency.

2/3. upload-cancel-confirm and sheet-escape interacted with server-rendered controls before
   Svelte hydrated them (caption fill lost -> cancel() navigates to /feed; custom role=radio
   / leave-button click no-ops). Wait for a post-hydration readiness signal (composer
   auto-focus / real profile name) before the first interaction — the same barrier the
   passing feed-based specs already have. (These are now belt-and-suspenders given ssr=false,
   but keep the specs robust regardless of render mode.)

Verified: mobile 22/22, and 0/50 full-mobile runs under load (was ~3/95 pre-fix).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/specs/09-mobile/sheet-escape.spec.ts

@@ -11,6 +11,13 @@ test.describe('Mobile a11y — sheets dismiss on Escape', () => {
     await signIn(page, g);
     await page.goto('/account');
 
+    // /account is server-rendered, so its controls exist and look clickable BEFORE Svelte
+    // hydrates their handlers. The data-mode options are custom `div[role=radio]` (no native
+    // input fallback), so a pre-hydration click does nothing and the sheet never opens (a
+    // reproduced ~1% flake). The profile name/expiry are filled in onMount (post-hydration);
+    // waiting for the real display name is a reliable "the page is interactive now" barrier.
+    await expect(page.getByText(g.displayName)).toBeVisible();
+
     // Click the "Original" radio in the Datennutzung section to open the warning sheet.
     // The radio's accessible name is its title + description ("Original Lädt die
     // Originaldateien…"), so anchor on the start, not the end.
@@ -33,6 +40,10 @@ test.describe('Mobile a11y — sheets dismiss on Escape', () => {
     await signIn(page, g);
     await page.goto('/account');
 
+    // Wait for /account to hydrate before tapping the (server-rendered) leave button, so the
+    // click reaches a live handler. See the test above.
+    await expect(page.getByText(g.displayName)).toBeVisible();
+
     await page.getByRole('button', { name: /Event verlassen/i }).click();
     const sheet = page.getByTestId('confirm-sheet');
     await expect(sheet).toBeVisible();