Add the Three.js constellation centerpiece and redesign the homepage

Data model: nodes are real content entries (writings, portfolio,
bookshelf), edges are shared-tag connections — the same relationship
"related posts" already uses, drawn instead of listed. No decorative
or fake data.

- src/lib/constellation-data.ts: builds the node/edge graph from the
  actual collections at build time.
- src/lib/constellation-scene.ts: vanilla Three.js scene (no
  framework) — fibonacci-sphere layout, auto-rotation, pointer
  raycasting with a title tooltip, click-through to the entry's page.
  Honors prefers-reduced-motion (skips rotation and the spring-eased
  entrance, which reuses src/lib/spring.ts) and checks WebGL support,
  falling back to a plain count for browsers/settings that lack it.
- src/components/Constellation.astro: wires it up, serializing the
  graph as inline JSON for the client script to consume.
- Homepage rebuilt around it: hero constellation above a 3-column
  dashboard (latest writings/portfolio/bookshelf), using a new `wide`
  layout variant on Base.astro (dashboard needs more than the 42rem
  article width).

Also: removes "life and beliefs" from all site copy (site.config.ts,
the llms.txt generator + its output) per request — description is
now "Computer science and mathematics."

Verified in-browser: real WebGL scene renders with correctly
positioned/colored nodes and edges, hover shows the right title via
raycasting, layout holds at the new wide container width.

apps/site/src/styles/global.css

@@ -99,6 +99,10 @@ pre {
   padding-inline: var(--space-5);
 }
 
+.container--wide {
+  max-width: 64rem;
+}
+
 .eyebrow {
   font-family: var(--font-mono);
   font-size: var(--font-size-sm);