docs(site): redesign landing and download for dev-tool aesthetic

[Tam-Leal]

Summary

• Redesign landing page (index.html) with a dev-tool aesthetic: IBM Plex Sans/Mono, subtle grid background, flat cards, terminal-style section labels, and app-aligned blue accent (#3b82f6)
• Remove hero particle canvas animation; keep logo, dashboard preview, and tentacles intact
• Sober icon treatment: neutral gray by default, blue accent on hover; problem/hub icons at reduced opacity
• Restructure download page: prerequisites before download, trust bullets inside card, dynamic OS title, file size from GitHub API, install-from-source block
• Extract download styles to site/css/download.css; update manifest.json theme colors

Type of change

• Bug fix (user-visible)
• Feature (user-visible)
• Documentation / site only
• Tests, CI, or refactor (no user-visible change)

Checklist

• I ran npm test locally (required for code changes)
• I smoke-tested with npm start if dashboard.html or API behavior changed
• No secrets in commits (config.json tokens, .env, PATs)
• Updated CHANGELOG.md under Unreleased when the change is notable

Release

• No GitHub Release needed (default for tests, docs, CI-only)
• Release needed — describe why users should download a new build:

Test plan

• Serve site locally: npx serve site
• Landing /: hero, tentacles, dashboard preview, stats grid, feature/problem/hub/security sections
• Download /download.html: OS detection, platform switch, version + size fetch, install steps, prerequisites order
• Mobile: landing preview scale, download card layout
• Verify no em-dashes in copy; blue accent only on CTAs, links, and hover states