[Bug] Docked plugin panes render opaque background, ignoring terminal transparency

Bug Description
Title: Docked plugin panes paint an opaque background; no way to let a translucent terminal background show through

Summary
In fullscreen mode, a docked plugin Pane is drawn with an opaque background. The main transcript is not. With a translucent terminal (for example Ghostty with background-opacity = 0.6), the chat area shows the terminal's background but the dock is a solid block.

Environment

• Claude Code 2.1.x, fullscreen, terminal at least 110 columns wide so the pane docks
• macOS, Ghostty with background-opacity = 0.6
• Plugin: glowup 0.9.0 (MIT), plus a bare test pane (below)

Repro

1. Set a translucent terminal background.
2. Make a minimal plugin whose ui.render hook for { component: 'Pane' } returns <Box><Text>hello</Text></Box>. It sets no colours and no backgroundColor.
3. Open the pane with $.ui.open({ id, title }).
4. The dock is opaque. The transcript beside it is translucent.

What I ruled out

• The plugin sets no background. The test pane above has none.
• /theme dark-ansi makes no difference.
• Custom themes accept only solid colour values (#rrggbb, ansi:<name> and similar). There is no alpha, and no "default" or "unset" value.
• PaneOpenArgs and the Pane props have no background or opacity option.
• I found no settings.json or fullscreen option for it in the docs.

Expected
Some way to let docked panes use the terminal's default background, any of:

• a PaneOpenArgs option (for example transparent: true)
• a theme colour value meaning "terminal default / unset" that the dock honours
• a setting that leaves the dock's cells with an unset background, as the transcript does

Why it matters
Users who run a translucent terminal get a mismatch: a see-through chat beside a solid sidebar. This affects any plugin that uses a docked pane, not just one plugin.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.292
• Feedback ID: 65838b4c-7219-43ce-9118-941567925ab4