Fix mobile model picker contrast and sizing

[ronak-guliani]

What does this PR do?

This is a follow-up to #8 that applies the missed mobile picker polish commit. It makes the model picker readable by adding an opaque inner surface over the glass/blur background and reduces popup width/height to 75% of the prior size. This keeps all existing picker functionality while improving legibility and visual balance.

How did you verify your code works?

I cherry-picked the missed commit onto latest origin/dev and reviewed git diff origin/dev... to confirm this PR only changes packages/mobile/src/components/model-picker.tsx.

[github-actions]

Hey! Your PR title Fix mobile model picker contrast and sizing 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.