Add toggleable dark theme + reduced-motion guard (TraitMech site)

[realmarcin]

Adds a user-toggleable dark theme (OS `prefers-color-scheme` default plus a `data-theme` toggle persisted to `localStorage`) and a reduced-motion guard to every generated TraitMech page, without changing the light appearance.

Base is `fix/embedding-clickthrough-404` (not main) because that open PR also touches umap.html/graph.html. Do not merge until the base lands.

What changed

• Vendored `pages/theme-toggle.js` — self-contained fixed ☾/☀ button; sets `data-theme` on `` before paint, themes from page tokens.
• `style.css` (single source → `pages/assets/style.css`): dark token remap under both `@media (prefers-color-scheme: dark)` and `:root[data-theme="dark"]`, plus overrides for hard-coded light colors (site-header, hero gradient, `.stat`, buttons, pathograph container/legend, graph-warning, edge-evidence), d3 scatter SVG legend-text fill / hover halo, and a reduced-motion guard. Covers browse / category / trait / umap / graph pages.
• `base.html`: includes `theme-toggle.js` at end of body → propagates to all base-derived pages on re-render.
• `index.html` (standalone landing): inline dark block mapping `--pastel-b → --bg` and `--pastel-a → heroA` (page/footer ground goes dark, hero fades to dark), dimmed translucent `.stat`/footer surfaces, darkened `.btn` text, reduced-motion guard, and the include.
• Re-rendered with `scripts/render_trait_pages.py` — 477/477 trait pages.

Palette (dark)

bg `#171417` · surface/card `#211d20` · ink `#efeaea` · muted `#ada4a6` · border `#332d30` · accent `#ef7377` · heroA `#33201f` · code-bg `#0d0b0d` · accent-soft `#2a1e1f`

Verification

• `theme-toggle.js` present at web root; 490/490 HTML pages include it once.
• `assets/style.css`: 2 `prefers-color-scheme: dark` blocks, 13 `:root[data-theme="dark"]` rules, 1 reduced-motion guard.
• `index.html`: inline dark block (both selectors) + reduced-motion.
• umap/graph SVG plot bg already uses `var(--card-bg)`; legend text fill + hover halo driven from tokens. HREF_BY_ID click-through fix left untouched.
• Additive only — base `:root` token blocks unchanged, light look identical.

Additive-only; DO NOT MERGE (stacked on `fix/embedding-clickthrough-404`).

🤖 Generated with Claude Code

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[realmarcin]

Reviewed the PR's own diff (base fix/embedding-clickthrough-404): base.html, index.html, style.css, vendored theme-toggle.js.

Clean — no issues, and correctly additive.

• Light theme is untouched: base :root token blocks are unchanged; all new rules are gated behind @media (prefers-color-scheme: dark) :root:not([data-theme="light"]) and :root[data-theme="dark"], so the default light look is byte-for-byte the same.
• theme-toggle.js applies data-theme before paint (no FOUC), persists to localStorage, and follows live OS changes only when no explicit choice is set.
• Include path uses {{ root }}theme-toggle.js in base.html (inherited by umap/graph/trait/category, which all {% extends %} it) and in the standalone index.html — resolves correctly at every depth (../../theme-toggle.js on trait pages, theme-toggle.js at root).
• Reduced-motion guard present in both style.css and index.html. HREF_BY_ID click-through untouched.