Use OpenInstinct logos across the app

[fmhall]

Replace the remaining cube branding with the OpenInstinct mark from openinstinct-web, including the app navigation, browser and Apple icons, and a shared social preview inherited by the vault route.

Allow anonymous requests to the three exact branding asset URLs so browsers and social crawlers can load them. Regression coverage keeps paths beneath those filenames protected.

Validation:

• pnpm check — all six tasks passed, including 731 tests across 84 files.
• pnpm build — passed.
• pnpm exec vitest run web/auth/tests/proxy.test.ts — 13 tests passed.
• Confirmed copied assets match the source byte for byte and the SVG mark matches on light, dark, and cream backgrounds.
• Local production smoke confirmed asset HTTP 200 responses, rendered favicon/Apple/Open Graph/Twitter metadata, and sign-in redirects for protected routes.
• Independent review completed with no remaining findings.