feat: add per-pair fee editor

[gaoflow]

Summary

• add inline per-pair fee editing on the Pairs page
• validate fee_bps as an integer from 0 to 1000 before PATCHing
• optimistically update the row, roll back on backend errors, and show success/error feedback

Closes #2

Tests

• npm test -- src/app/pairs/page.test.tsx --runInBand
• npm run lint
• npm test -- --runInBand
• npm run build
• NEXT_PUBLIC_STABLEROUTE_API_BASE=http://localhost:3011 npm run build
• Rendered smoke: started a temporary mock API on 3011 and production Next server on 3010, then captured /pairs with Playwright CLI.

Browser runtime fallback: the in-app Browser was available as a skill, but the runtime returned Browser is not available: iab; I used Playwright CLI screenshots instead.

Screenshot evidence saved locally:

• /tmp/stableroute-pairs-fee-editor.png
• /tmp/stableroute-pairs-fee-editor-after-update.png

[mikewheeleer]

appreciate the PR @gaoflow 🙏 heads up though — #2 wasn't assigned to you, and we merge from the assigned contributor to keep it fair. please claim an unassigned issue first, then PR. closing for now 🙌