Add pre-flight credit check before PR creation on coverage dashboard

[hiroshinishio]

Summary

• Checks credit balance before creating PRs on the coverage dashboard, blocking users with insufficient credits
• Subscription users (e.g. SpiderPlus) bypass the check; credit users must have $7 * numPRs available
• Shows an error modal with current balance, required amount, and a link to the credits page
• Changed Modal component message prop from string to ReactNode to support the link

Social Media Post (GitAuto)

A user with $0 balance created two PRs from our dashboard before getting blocked by the backend credit check. The PRs already existed on GitHub by then. Now the website checks credits upfront - if you can't afford it, you see a modal with your balance and a link to buy credits. No wasted PRs.

Social Media Post (Wes)

Found a gap in our payment flow. Users could click "Create PRs" on the coverage dashboard with zero credits. The backend would reject it, but the PR was already on GitHub. Moved the credit check to the frontend - now it blocks before anything hits the API. Simple fix that should've been there from day one.