Use Geist for UI text and Geist Mono for code text

[ronak-guliani]

What does this PR do?

This updates the app typography defaults so non-code UI text uses Geist and code/file-related text uses Geist Mono. It switches shared theme font tokens to Geist/Geist Mono, adds Geist Mono as a selectable font option, and makes it the default appearance font. It also updates i18n font option labels and the related e2e assertion so tests match the new default behavior.

How did you verify your code works?

I reviewed the full diff against origin/dev and confirmed all font wiring points (theme tokens, font-face loader, settings defaults/options, and e2e expectations) are internally consistent.

[github-actions]

Hey! Your PR title Use Geist for UI text and Geist Mono for code text doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.