feat: add transparent background toggle functionality

[AlexNaga]

🚀 What does this PR do?

Adds a toggleable transparent background setting to the TUI.

🔍 Current behavior

The TUI always renders its own background color from the active theme.

✨ New behavior

• New "Transparent background" toggle in Settings (Ctrl+P > Settings)
• When enabled, the TUI uses the terminal's native background color instead of the theme background
• Off by default to avoid the first-time startup flash
• Setting is persisted to config.json

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📝 Other information

• Terminal background is detected via renderer.getPalette()