Add first-run welcome guide modal

[Adamchaua]

Fixes #13

What changed

• Added a first-run Welcome modal on the trading dashboard.
• The modal briefly explains CAP, the dashboard flow, Arbitrum funding, and risk/fees.
• Added bridge/docs CTAs and a reusable Guide nav entry so users can reopen the instructions later.
• Persisted the first-run behavior via existing user settings so experienced users are not blocked repeatedly.

Validation

• npm run build
• Browser smoke test at http://127.0.0.1:8080/trade confirmed the welcome modal appears for a fresh local session.
• Clicked Got it and confirmed the modal closes.
• Browser screenshot proof: /home/ckelvinkhanh/.hermes/cache/screenshots/browser_screenshot_e9537de988cd4949a0be86ebf20df4ff.png

Notes

I am requesting assignment/consideration for the bounty on this issue. Payout details can be provided after maintainer confirmation.

[Adamchaua]

CI note: the only current check is Vercel, and it links to "Account is blocked" with no project build output. Local validation for this PR already passed with npm run build, plus a browser smoke test confirmed the first-run modal opens on /trade and closes via Got it.