Demo polish: tighter onboarding + flip icon ladder to favicons-first

Two independent UX fixes visible on the public demo:

apps/compliance-cockpit/src/components/onboarding/scenario-picker.tsx
- Was: 4 large cards, each with a paragraph blurb (~30 words),
  large icons, 4-column grid feel. Header line was long, subtitle
  redundant.
- Now: 4 compact rows (2×2 on desktop, stacked on mobile), one-line
  disambiguating blurb each ("Python: Anthropic, OpenAI, LangChain,
  CrewAI, Gemini, Mistral"). Header shortened; subtitle trimmed to
  a single line. Reads in 2-3 seconds; click confidence goes up.

apps/compliance-cockpit/src/components/ui/brand-icon.tsx
- Was: Simple Icons primary → icon.horse fallback. Gave brand-color
  monochrome marks — Gmail rendered as red disc + white envelope,
  not the multi-color M users know.
- Now: icon.horse primary → Simple Icons fallback. Fetches each
  site's actual favicon, which for Gmail/Google/Meta/YouTube is
  the multi-color mark; for Anthropic/Stripe/Vercel is still the
  clean colored logo. Simple Icons steps in only when a favicon
  is unreachable AND the brand has a Simple Icons slug.
- Rationale: favicons are the source of truth for what people
  recognise as "the brand". Simple Icons is technically nicer
  vector but wrong colour palette for a big chunk of the top
  brands the Cockpit surfaces.