feat: add interactive bounty flow diagram

[dannyward630]

Summary

• Add an interactive SVG bounty lifecycle diagram to the How It Works page covering post -> claim -> work -> submit PR -> review -> payment.
• Support hover, focus, click, and keyboard activation with stage-specific explanatory copy and aria-pressed state.
• Add a focused Vitest suite for rendering, click interaction, and keyboard activation.
• Narrow the root Python lib/ ignore rule so the existing frontend src/lib imports can be committed, then add the small helper modules those imports already expected.

Closes #851

Verification

• npm run test -- bounty-flow-diagram.test.tsx
• npm run build
• git diff --check
• Browser smoke: opened /how-it-works on Vite at 127.0.0.1:5174, clicked the Review stage, confirmed the tooltip and aria-pressed state update, and checked a 320x568 viewport for no horizontal overflow.

Notes

• I also tried the full npm run test; the current main branch test suite still fails before reaching this component because several baseline test files import missing packages/components/backend files such as @playwright/test, backend/app/api/og.py, admin/tokenomics/reputation modules, and others. The new focused test passes and the production build is green.
• Local dev smoke shows /api/stats proxy errors when no backend is running; that appears unrelated to this UI component.
• I did not include a wallet address in this PR body; happy to add one if the bounty process requires it.