feat: confetti celebration UI on application submit and acceptance

[balebbae]

Add a celebratory animation (confetti or similar) at the two high points of the hacker flow:

1. On application submit — after a successful submit, when the user lands on the confirmation/status view.
2. On acceptance — when a user with an accepted status views their status page.

Notes / considerations

• Should feel like a moment, not a gimmick — matches the bold black/white Nike-style hacker design language.
• Fire once per event, not on every render or every revisit to the page. Needs some persistence (localStorage or a server-side "seen" flag) so returning to the status page doesn't re-trigger it.
• Respect prefers-reduced-motion — skip or substitute a static treatment.
• GSAP is already available in the project and is a reasonable fit; a lightweight dedicated confetti lib is also an option.

Open questions

• Do we want distinct treatments for submit vs. acceptance, or the same effect reused?
• Should rejection/waitlist get any (non-celebratory) counterpart, or stay untouched?