feat(docs): add custom 404 page with playable Meteor mini-game

Override the Docus error page with a custom 404 that mounts a canvas-based,
meteor-shooting arcade mini-game themed around the Meteor design system.

- error.vue: full-viewport game below the site chrome; SSR renders an
  accessible "Page not found" hero (revealed via noscript for no-JS), the
  game fades in on the client with no static-page flash.
- Meteor404Game.vue: pure 2D canvas, fixed timestep, pooled entities, colors
  read from the --ui-* design tokens so it follows light/dark mode.
- Playable on desktop (rotate/thrust/fire, screen wrap, destructible "404"
  glyphs, difficulty ramp, arcade pause/game-over menus, share-score copy);
  ambient background on touch/small viewports.

Author: fabianhueske