feat: Bounty Countdown Timer Component (#826)

[CHY9213]

Summary

Implements #826 — Bounty Countdown Timer Component (T1, 150K FNDRY).

What it does

• Real-time countdown (days/hours/minutes/seconds) that updates every second
• Color-coded urgency: normal -> warning (< 24h) -> urgent (< 1h)
• Shows "Expired" when deadline passes
• Two variants: minimal (for BountyCard) and full (for BountyDetail sidebar)
• Visual urgency indicators (icon changes from Clock to AlertTriangle)

Files changed

• New frontend/src/components/bounty/CountdownTimer.tsx -- the countdown component
• Modified BountyCard.tsx -- uses CountdownTimer instead of static text
• Modified BountyDetail.tsx -- uses CountdownTimer (full variant) in sidebar

Behavior

State	Minimal	Full
>24h left	"3d 12h left" muted	"3d 12:34:56" muted
<24h left	"18h 30m left" yellow	"18:30:45" yellow
<1h left	"45m 12s left" red	"00:45:12" red + URGENT
Expired	"Expired" red	"Expired" red

Closes #826

Wallet: HUFz3mnXkSDzfxfRgiKsZ6w5zgfcwShigHrYGxvgrjAF