#73 Code view: add build-time syntax highlighting (no JS) + line-number gutter + file toolbar

The file view has zero token coloring — static/style.css contains no highlight rules (grep -c tok- static/style.css = 0) and file pages emit plain <span class="line"> text. A code mirror without highlighting is the single biggest "worse than GitHub/GitLab/Forgejo" gap.

Current file view

• files/src/build.rs.html: flat grey monospace, 13px/19px, with a floating line-number gutter that has no separator rule and uses --muted (2.76:1 dark / 4.26:1 light).
• h1 repeats the breadcrumb path (root / src / build.rs then <h1>src/build.rs</h1>).
• Meta bar: src/build.rs · 94.3 KiB · 2333 lines · Raw with "Raw" as plain text.
• No wrap toggle, no copy button, no syntax hints.

Scope

• Tokenize at build time (no JS, no deps beyond what we have) for a small language set: Rust, JS/TS, JSON, YAML, TOML, Markdown, shell, HTML/CSS, Python, Go. A ~200-line lexer producing tok-kw/tok-str/tok-num/tok-com/tok-fn/tok-type spans is enough; unknown extensions stay plain.
• Tokyo Night palette: --tok-kw (purple/magenta), --tok-str (green), --tok-num (orange), --tok-com (muted italic), --tok-fn (blue), --tok-type (cyan). Must pass contrast in both themes.
• Gutter: sticky line-number column, --fg-dim, right-aligned, 1px --border rule between numbers and code, user-select:none, anchors preserved (id="L42").
• File toolbar: breadcrumb (drop the duplicated h1, or keep a short build.rs), Raw as a proper secondary button, size + line count as dim chips, and a "wrap" <details>-free toggle is impossible without JS — so instead: white-space:pre + horizontal scroll, plus a <a href="/raw/…"> for full text.
• Keep output byte-budget: tokens add ~15–30% to file page size; measure on src/build.rs (94 KiB → ~120 KiB) and confirm acceptable, or cap highlighting to files < N KiB.

Acceptance

• Rust/JS/JSON/YAML/TOML/MD/shell files render colored tokens
• unknown extensions and huge files stay plain (no regression)
• line anchors still work and are selectable/shareable
• contrast of every token color ≥ 4.5:1 in both themes
• build-time cost stays negligible (no new deps)