Fix OSC 11 background color reporting

[uadhran]

Summary

Fixes the terminal background color reported through OSC 11 [ #10121 ].

Tabby was sometimes passing a transparent/null background through to xterm when the terminal was following the app theme. In that case, OSC 11 could report black instead of the actual background color.

This change keeps the existing host background behavior, but makes sure xterm always receives a concrete background color for its internal theme.

Changes

• kept getTerminalBackgroundColor() for the outer terminal container
• added a separate helper for the xterm theme background
• updated the xterm frontend to always use a resolved background color

Testing

• linted the touched files with eslint
• built tabby-terminal successfully with webpack

Notes

This should mainly affect background color reporting and similar xterm internals. The visible terminal background behavior should remain unchanged.