feat(tui): add light color scheme with auto-detect and JSON config

[dragonee]

What & why

The TUI ships a single hardcoded, dark-only "Blade Runner amber" theme that is unreadable on light/white terminal backgrounds. This adds a second, light-optimized scheme while keeping the existing dark one byte-for-byte unchanged.

How

• Palette → lipgloss.AdaptiveColor{Dark, Light} (internal/tui/view.go). The Dark variants are the original hex (dark output is identical to before); the Light variants are a warm palette tuned for contrast on white. The duplicated inline #1a1305 (9 sites) is folded into colOnAccent / colSurface0 so badge/tab text stays legible in both modes.
• Non-palette subsystems get the resolved dark/light flag via a new internal/tui/theme.go (SetTheme): glamour markdown (dark/light standard style), chroma syntax highlighting (dark vs. light style list), and the spinner color.
• Startup resolution (cmd/prcheck/main.go): the scheme is resolved once, before Bubble Tea enters AltScreen, and frozen with lipgloss.SetHasDarkBackground. The auto path detects the terminal background there — sidestepping the AltScreen OSC-query deadlock documented in markdown.go — and falls back to dark when the terminal doesn't answer.
• Config (internal/config): new PRCHECK_THEME env var (auto|dark|light, default auto) plus an optional JSON config file at os.UserConfigDir()/prcheck/config.json, parsed with stdlib encoding/json. Env overrides the file; a missing/malformed file is non-fatal.

Notes

• No new dependency (stdlib encoding/json; glamour/chroma/lipgloss already vendored).
• Default auto means existing users on dark terminals see no change.

Testing

• go build ./..., go vet ./..., gofmt clean.
• New internal/config/file_test.go: file/env precedence, malformed-file tolerance, theme normalization.
• All TUI tests including goldens pass. Verified the palette switches deterministically: dark renders #ffb000, light renders #b25e00, toggled by SetHasDarkBackground.

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