fix(ui): silence the two benign cancelled-transition rejections

Round-4 console hygiene: fast navigation rejects with "AbortError:
Transition was skipped" (the browser's MPA view transitions —
@view-transition navigation:auto — when a navigation interrupts a
running crossfade) and Alpine's x-transition hide rejects with
{isFromCancelledTransition: true} when a re-show supersedes an
out-phase. Both are races the user cannot lose anything in, but they
print console errors on every quick click — and the knob matrix's
zero-console-error tripwire (the thing that catches silent CSP death)
depends on a clean console. A narrow global gate swallows exactly
these two shapes; every other rejection still reports.

web/src/main.ts

@@ -42,5 +42,30 @@ import "./unlinked_books";
 import "./websocket";
 import "./woodPanelingInit";
 
+// ─── cancelled-view-transition noise gate (plan round-4 item 9) ────
+// Two transition races are BENIGN BY DESIGN and reject their promises
+// as unhandled rejections on every fast navigation:
+//   - `AbortError: Transition was skipped` — the browser's MPA view
+//     transitions (@view-transition { navigation: auto } in input.css)
+//     when a new navigation interrupts the running crossfade.
+//   - `{isFromCancelledTransition: true}` — Alpine's x-transition hide
+//     cancelled by a re-show before the out-phase finished.
+// Both mean "an animation was superseded", nothing was lost; the knob
+// matrix's zero-console-error tripwire depends on the console staying
+// CLEAN, so only these two shapes are swallowed — real rejections
+// still report.
+window.addEventListener("unhandledrejection", (event) => {
+  const reason = event.reason as
+    | (Error & { isFromCancelledTransition?: boolean })
+    | null
+    | undefined;
+  const cancelledTransition =
+    reason?.isFromCancelledTransition === true ||
+    (reason?.name === "AbortError" && /transition was skipped/i.test(reason.message ?? ""));
+  if (cancelledTransition) {
+    event.preventDefault();
+  }
+});
+
 armAuthFormRedirects();
 Alpine.start();