♻️ fix(home): complete reset + new strategy — separate portal.css

STRATEGY: Instead of modifying extra.css (which broke text visibility 9 times),
create a NEW portal.css that only styles the hero/section portal elements.
extra.css is restored to its original working state (1353 lines).

Files:
- docs/stylesheets/portal.css — NEW: hero + section styles only (no color vars)
- docs/javascripts/particles.js — particle starfield + logo morphing
- docs/overrides/main.html — canvas injection only
- docs/index.md/.en.md — redesigned hero page with sections
- mkdocs.yml — register portal.css + particles.js + updated nav

Author: ouyangyipeng