fix(home): restart the hero banner timer on a manual change

The rotation interval was installed once when the banner mounted and never
touched again, so a swipe, arrow or dot tap inherited whatever was left of the
running countdown — swiping 5.5s into a 6s interval moved the banner on half a
second later.

The timer moves into heroRotation.ts as a small restartable object so it can be
unit-tested, and every manual navigation path restarts it from that moment.
Verified red-first: with restart() reverted to leave a running timer alone, the
regression test fails.

Release 0.9.1.

src/lib/components/home/heroRotation.ts

@@ -0,0 +1,40 @@
+// Auto-rotation timer for the home-screen hero banner.
+//
+// Extracted from HeroBanner.svelte so it can be unit-tested: a manual swipe or
+// dot/arrow click must restart the countdown from that moment. The old code
+// installed one bare setInterval and left it running, so swiping late in an
+// interval made the banner jump to the next item almost immediately.
+//
+// TRACES: UR-034 | DR-038 | UT-207
+
+export interface RotationTimer {
+  /** (Re)start the countdown from now, replacing any pending tick. */
+  restart(): void;
+  /** Cancel the countdown. */
+  stop(): void;
+  isRunning(): boolean;
+}
+
+/**
+ * Create a repeating timer that calls `onElapse` every `interval` ms once
+ * started. Restarting is idempotent — there is never more than one live timer.
+ */
+export function createRotationTimer(interval: number, onElapse: () => void): RotationTimer {
+  let handle: ReturnType<typeof setInterval> | null = null;
+
+  function stop() {
+    if (handle !== null) {
+      clearInterval(handle);
+      handle = null;
+    }
+  }
+
+  return {
+    restart() {
+      stop();
+      handle = setInterval(onElapse, interval);
+    },
+    stop,
+    isRunning: () => handle !== null,
+  };
+}