feat(tui): system-like custom themes

[nihil2501]

Issues

• [FEATURE]: Custom themes can use calculated system colors anomalyco/opencode#6322
• Can't use custom themes that use ANSI colors anomalyco/opencode#4236
  

Feature

Custom themes can now reference the user's terminal color palette by using "system" as a color value:

{
  "theme": {
    "background": "system",
    "foreground": "system",
    "primary": "#ff5500"
  }
}

Commits

Three atomic commits build up to this feature:

1. feat(tui): system-like custom themes
2. refactor(tui): theme switching & loading reactivity
3. fix(tui): system theme light & dark variants

This can be split into 3 sequential PRs if preferred, one per commit.

[github-actions]

Hey! Your PR title System theme colors 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.