feat: add warm-cream light mode with system + manual toggle

[avrabe]

Summary

For summer / direct-sunlight visibility. Cream-paper aesthetic rather than a flat invert — the dark mode here is cool/blue, so cream gives strong visual differentiation and reads cleanly in glare.

Resolution order:

```
data-theme="dark" → dark always
data-theme="light" → light always
no data-theme + prefers-color-scheme: light → light
otherwise → dark
```

Manual toggle (sun/moon button in nav) persists choice via localStorage; OS auto-switch stops applying once a manual choice exists.

How it's wired

• CSS custom properties as the source of truth. 18 theme tokens (`--bg`, `--surface`, `--text`, `--accent`, etc.) on `:root`, with `:root[data-theme="light"]` and `@media (prefers-color-scheme: light)` overrides. Sass variables (`$bg`, `$text`, ...) now alias into these, so all existing partials keep compiling without per-file changes.
• One `lighten()` call had to be replaced with a dedicated `$accent-hover` token (Sass color functions can't operate on `var()`).
• Inline bootstrapper in `` applies the saved preference before render to avoid flash-of-wrong-theme.
• Mermaid mirrors the palette in JS — light variant uses `theme: 'default'` (not `'base'`, which left derived colors falling back to dark and produced black-blob nodes).

Light palette

• bg `#f4efe4` (cream paper) → faint dot-grid `rgba(68,81,106,0.10)` replaces the animated canvas
• text `#1a2235` (deep navy)
• accent `#3a5fce` (darkened from `#6c8cff` for AA on cream)
• glass cards become opaque cream surfaces (translucency reads poorly on light)

Known follow-ups (not in this PR)

• Mermaid diagrams require a page reload to re-render after toggle (the rendered SVG has lost the original source). Live re-render is doable but more code than the use-case demands.
• `templates/docs-styles.html` still has hardcoded colors. Docs nav is hidden so low priority.
• `templates/projects.html` (cube revamp) and `content/projects/_index.md` are WIP and stay local. The cube template already got light-mode fixes locally so it's ready when the projects revamp ships.

Test plan

• zola build clean (23 pages)
• Locally tested in light mode: landing, blog post with mermaid, status issue style references, projects cube (local-only)
• User-confirmed visually
• CI passes
• After merge: deploy succeeds, light mode visible at https://pulseengine.eu via toggle button or by setting `prefers-color-scheme: light` in the browser

🤖 Generated with Claude Code