Refine DSA practice workspace proportions

[Gotnochill]

Summary

• give the solution editor a wider 55% default share while keeping the draggable, persisted split
• reduce the practice card's horizontal padding and the center divider spacing
• preserve the existing pane order, controls, responsive stacking, borders, and color scheme

Testing

pm run typecheck

pm test (112 tests)

px prettier --check app/globals.css components/practice/problem-workspace.tsx tests/e2e/problems.spec.ts

• Focused Playwright layout tests could not start because local Postgres was unavailable at localhost:5432; CI can run them with its database service.

Fixes #123

[vercel]

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