[codex] Refresh public landing auth entry

[Egonso]

What changed

• Reworks the public auth/landing entry into a wider, evidence-first layout with a Use Case Pass preview.
• Adds German/English marketing copy for the new hero, process section, and sample PDF block.
• Adds a landing-only language switcher and preserves query strings when switching locales.
• Updates logo/favicon assets used by the public entry.
• Updates the launch-flow source assertion to match the new copy.

Notes

• The existing global support widget remains unchanged. On narrow mobile viewports it can still sit over the lower edge of content, which is a broader GlobalChrome decision rather than part of this landing copy/layout scope.

Validation

• npm run typecheck
• npm run lint
• npx tsx --test src/lib/launch/core-launch-flows.test.ts
• Playwright screenshots for http://localhost:9002/de on desktop and mobile.

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