Add transparency/translucent background support for TUI

[JosXa]

Summary

Add support for transparent/translucent terminal backgrounds in the TUI, allowing the terminal's background to show through opencode.

Motivation

Many users prefer translucent terminal windows for aesthetic reasons or to see content behind the terminal. Currently, opencode renders with an opaque background that overrides any terminal transparency settings.

Proposed Solution

• Add a toggle command in the TUI to enable/disable transparency
• When enabled, set the background color to fully transparent (RGBA 0,0,0,0)
• Persist the setting so it's remembered across sessions

Additional Context

This feature has been requested and implemented in PR #5657.

[github-actions]

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

• System theme doesn't apply Ghostty transparency on load (v1.1.10) #8480: System theme doesn't apply Ghostty transparency on load (v1.1.10) - Closed, but related to transparency support
• System theme doesn't apply Ghostty transparency on load (v1.1.10) #8477: System theme doesn't apply Ghostty transparency on load (v1.1.10) - Duplicate/related transparency issue
• UI background is not transparent/adaptive in vscode #8399: UI background is not transparent/adaptive in vscode - Different component but same root cause (background transparency)
• Invisible text in question selector #7739: Invisible text in question selector - Shows issues with transparent backgrounds in current implementation
• No text on transparent background in dialog-alert, dialog-confirm & dialog help buttons #9920: No text on transparent background in dialog-alert, dialog-confirm & dialog help buttons - Specific case of transparency issues
• Text selection has poor contrast/readability on light terminals with certain themes #5295: Text selection has poor contrast/readability on light terminals - Related to background/theme rendering issues

Feel free to ignore if none of these address your specific case.