Adds a minimal hand-rolled highlighter for ts/js/jsx/tsx, py, sh/bash, go, rust,
json, yaml, sql. Recognizes whole-line comments, single/double/backtick strings,
numbers, and per-language keyword sets. Unknown langs fall through to the current
plain rendering; the existing diff-specific colorization is preserved.
Closes the §8 "Markdown syntax highlighting is missing (only diff gets colored)"
finding from the TUI v2 audit without pulling in a highlighter library.
