fix(desktop): declare minimum Rust version 1.88

[rubencu]

Issue for this PR

Closes #23611

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This declares the desktop crate's minimum Rust version as 1.88 in packages/desktop/src-tauri/Cargo.toml and documents that requirement in the desktop README.

The desktop build already needs a newer compiler than 1.85.1, but without a crate-level minimum it fails with dependency-level errors instead of a direct Cargo message that the desktop crate itself requires a newer Rust toolchain.

How did you verify your code works?

• cargo +stable check in packages/desktop/src-tauri now fails with a direct opencode-desktop requires rustc 1.88 error on rustc 1.85.1
• cargo +1.88.0 check in packages/desktop/src-tauri succeeds
• repo typecheck passed during commit/push

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title [codex] pin desktop Rust toolchain to 1.88.0 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.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍