fix(review): show diff colors on non-truecolor terminals, fix tab indentation, add commit picker

[reckerp]

Why

gx review had two rendering bugs that made the diff hard to read, and the scope switcher was incomplete:

1. No syntax highlighting on many terminals. The widget emitted 24-bit truecolor (38;2;R;G;B) for every syntax color and every add/remove background. Terminals that drop truecolor escapes — tmux and macOS Terminal.app unless specially configured for RGB/Tc — silently ignored them, so the diff rendered as plain, uncolored text. (Highlighting was working; the terminal just never saw the colors.)
2. Broken indentation. Tabs were passed through as a single cell, so any tab-indented file lost its indentation.
3. Missing scope options. There was an s switcher (branch / +worktree / uncommitted) but no way to pick a specific commit or range, and it was under-discovered.

What changed

• Color-depth adaptation (new src/ui/review/color.rs): detect whether the terminal reliably supports truecolor, and downsample every RGB color (syntax foregrounds + diff palette) to the nearest xterm-256 index otherwise. Conservative auto-detection treats tmux/screen as 256-color unless truecolor = "always".
• Tab expansion: tabs render as tab_width-aligned spaces in the diff (byte offsets stay aligned so word-level emphasis is unaffected).
• Scope switcher rework: clearer s menu (uncommitted · all branch changes · branch + working tree) plus a commit picker — ⏎ reviews one commit, space marks a range endpoint to review older^..newer. Each scope keeps its own comments. Discoverable via the empty-state hint, help bar, and help overlay.
• Config: new [review] truecolor (auto / always / never) and tab_width (default 4). Documented in docs/review.md.

Testing

• cargo test --bin gx — 266 pass (incl. new color-downsampling and tab-expansion tests); cargo clippy clean.
• Captured the real binary's TTY output under tmux before/after: before emitted 38;2;… truecolor (dropped by tmux); after emits 38;5;N/48;5;N 256-color that tmux renders. Verified tab-indented lines now align with space-indented ones via a rendered-buffer dump. Smoke-tested the commit picker end-to-end (no panic).