feat(tui): allow the command palette background to differ from other dialogs

[hope-999]

Issue for this PR

Closes

#50901 — allow the command palette background to differ from other dialogs

Type of change

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

What does this PR do?

Fixes #50901.
The command palette previously shared the dialog surface with every other dialog. This meant that themes with transparent dialogs could not give the command palette its own background without also making all other dialogs opaque.
This change adds an optional palette theme surface. The command palette opens using this surface, while other dialogs continue to use dialog. If a theme does not define @palette, it falls back to the existing @dialog surface, so existing themes keep their current appearance.
Theme authors can now configure it like this:
"@palette": {
"background": {
"base": "#202040"
}
}

test file:
palette-test.json

How did you verify your code works?

• Ran the Command Palette test:
  bun test test/cli/tui/command-palette.test.tsx
• Ran the theme resolution tests:
  bun test test/theme/v2/resolve.test.ts
• Ran TUI and theme typechecks.
• Ran the full repository checks before pushing:
  bun run check
• Manually tested a custom theme with @palette.background.base set to a bright color. The Command Palette used the custom background, while other dialogs continued using the normal dialog background.

Screenshots / recordings

Checklist

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

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The following comment was made by an LLM, it may be inaccurate: