Make the celebration look like the library's realistic-look and fireworks examples

The celebration works now (#01M5Z00VJ6215NFZ8F3KJ48FXA, merged), but it looks wrong: 120 particles at scalar: 1 from a single point read as a small puff on a full-width page. The point of the effect is that landing a change request feels like something.

Scope

Use two of canvas-confetti’s own published examples, in the order they are meant to be read:

• the realistic look preset for the opening burst: particleCount: 500, spread: 180, ticks: 400, gravity: 0.5, decay: 0.92, startVelocity: 30, scalar: 1.2;
• the fireworks volley from both edges (angle: 60 at x: 0, angle: 120 at x: 1, spread: 55), with the count tapering as its clock runs out.

The example’s volley runs for 15 seconds. Ours has to stop much sooner: the page it lands on is a change request someone is about to read, so it should last about a second, not a quarter of a minute.

Acceptance criteria

• Landing a change request draws a burst that fills the viewport, and the volley ends on its own within roughly two seconds.
• The animation holds ~60fps while it runs, and no canvas is left behind once it is over.
• prefers-reduced-motion: reduce still yields the page and no animation.
• The CSP is unchanged, and the instantiation stays on the main thread (useWorker: false) — the worker path is what made the burst invisible before.