[FEATURE]: add built-in transparent TUI theme

[shkumbinhasani]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add a built-in transparent TUI theme that uses none for background, backgroundPanel, backgroundElement, and backgroundMenu so OpenCode can inherit terminal transparency when users want that behavior.

This keeps existing themes unchanged and gives users an explicit opt-in via /theme or config.

Context: implementation PR is #14563.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Add transparency/translucent background support for TUI #11866: "Add transparency/translucent background support for TUI" — open issue requesting the same transparent background feature
• [FEATURE]: Suggestion to set OpenCode TUI default background to transparent to match terminal themes. #14382: "[FEATURE]: Suggestion to set OpenCode TUI default background to transparent" — recently closed issue requesting the same transparent TUI background
• Use terminal theme colors by default #101: "Use terminal theme colors by default" — related open issue about blending with terminal colors

Please review these issues to see if your request is already covered. If your feature is distinct, feel free to clarify the differences.