Fix syntax theme previews to use normal option initialization

[pederbe]

Summary

--show-syntax-themes parsed command-line options directly, bypassing Git configuration, environment settings, and computed rendering options. Initialize preview options through the normal path so backgrounds, line filling, and empty-line markers match ordinary rendering with the selected syntax theme.

Add ANSI-output comparisons for dark and light themes, true-color modes, and a Git-config feature with environment-based color detection.

Fixes #2175.

Testing

• Both new regression tests pass. The output comparison failed with the old initialization.
• cargo test --locked: 416 passed, 1 failed, 8 ignored. The Windows path-separator failure in subcommands::external::test::subcommand_rg also reproduces on unchanged upstream HEAD.
• cargo clippy --locked --all-targets and cargo fmt -- --check pass. Clippy reports warnings outside the changed code.
• cargo build --locked --release passes. Command-line checks confirm matching ANSI output for dark/light themes, both theme groups without an explicit mode, and a custom config; the built-in sample also renders successfully.
• C dependencies were built with CFLAGS=-g0 to avoid a MinGW 8.1 debug-information compiler crash.
• Ubuntu 26.04 under WSL 2: 440 unit tests passed, 8 ignored. The deprecated-option and pager-history integration scripts pass, as do Clippy, formatting, and the release CLI checks.
• Linux make test stops at the raw-output/history comparison because binary-file notices are omitted. The same failure reproduces with unchanged upstream source. The other two shell scripts were run separately.