feat: add Dutch auction rate ticker

[newmattock]

Fixes #71.

Summary

• Add Dutch auction rate utilities that derive current bps from elapsed ledger-time approximation.
• Add an AuctionRateTicker component with current rate, minimum-rate target time, animated progress bar, and next-decrease countdown.
• Integrate the ticker into the LP invoice discovery/watchlist discount column while preserving the standard fixed-rate badge for normal invoices.
• Preserve optional auction metadata from invoice reads for auction_mode, start/min rates, and auction start/end timestamps.

Validation

• npm test -- src/utils/__tests__/dutchAuction.test.ts src/components/__tests__/AuctionRateTicker.test.tsx --reporter verbose
• npm run build
• git diff --check
• Browser smoke: http://127.0.0.1:3012/lp returned 200 and rendered the LP dashboard surface.

Notes

• Focused ESLint on LPDashboard.tsx/src/utils/soroban.ts still reports pre-existing errors in those files unrelated to this change, so lint is not a clean gate on current main.