Configure jjui theme for readability with Pencil Dark terminal theme (flakes-p7hc)

Add settings.ui.colors overrides to programs.jjui in modules/home/programs/jujutsu.nix to fix readability issues with the Pencil Dark (Gogh) WezTerm color scheme. The default dark theme uses ‘bright black’ for dimmed text and selected backgrounds, which is too dark/invisible in Pencil Dark.

Summary of Changes

Added settings.ui.colors overrides to programs.jjui block in modules/home/programs/jujutsu.nix. Key fixes:

• dimmed: "bright black" → "#808080" (mid-gray, readable in Pencil Dark)
• selected / revisions details selected bg: "bright black" → "#3a3a3a"
• revset completion bg: "black" → "#1a1a1a"
• revset completion dimmed: same fix as dimmed

Config verified at ~/.config/jjui/config.toml after just activate-home.