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/helpers/touch.ts

@@ -34,6 +34,47 @@ export async function longPress(page: Page, locator: Locator, durationMs = 600)
   await page.mouse.up();
 }
 
+/**
+ * A sub-threshold press dispatched ENTIRELY in-browser: `pointerdown`, an in-browser
+ * `setTimeout(holdMs)`, then `pointerup` — all on the element carrying `use:longpress`.
+ *
+ * Why not `longPress` (page.mouse + waitForTimeout) for the "quick tap must NOT long-press"
+ * case: those are three independent CDP round-trips with a Node-side wait between the down
+ * and the up. Under cross-test load the browser can process the `pointerup` far later than
+ * `holdMs` of *browser* time, so the app's real 500 ms long-press timer fires and the
+ * ContextSheet opens on what the test intended as a quick tap. (Observed: a 200 ms-intended
+ * tap opening the sheet ~1 % of full-suite runs.) Dispatching both events from one
+ * `evaluate` puts the down→up gap and the app's 500 ms timer on the SAME clock: even if the
+ * event loop stalls, the earlier-scheduled `pointerup` (holdMs) still fires before the
+ * later 500 ms timer, so the relationship holds. Mirrors `doubleTap`.
+ *
+ * No trusted `click` follows a synthetic `pointerup`, so use this ONLY where the assertion is
+ * that the long-press did NOT fire — not where a real click must land (see `longPress`).
+ */
+export async function quickTap(locator: Locator, holdMs = 200) {
+  await locator.evaluate((el: HTMLElement, ms: number) => {
+    const rect = el.getBoundingClientRect();
+    const x = rect.left + rect.width / 2;
+    const y = rect.top + rect.height / 2;
+    const mk = (type: string) =>
+      new PointerEvent(type, {
+        bubbles: true,
+        cancelable: true,
+        clientX: x,
+        clientY: y,
+        pointerType: 'touch',
+        isPrimary: true,
+      });
+    el.dispatchEvent(mk('pointerdown'));
+    return new Promise<void>((resolve) =>
+      setTimeout(() => {
+        el.dispatchEvent(mk('pointerup'));
+        resolve();
+      }, ms)
+    );
+  }, holdMs);
+}
+
 /**
  * Two rapid pointer-event pairs within the doubletap action's 300 ms window.
  *