Extract a shared Branding component and make logos optional

[Kaushik4141]

Footer.tsx, RatingScreen.tsx, ProgressScreen.tsx, and CompletionScreen.tsx each contain the same duplicated three-sponsor array (COSC / SOSC / Sceptix) and the same DK24 "Powered By" block — roughly 120 lines of copy-paste.

• Extract one <Branding /> component and use it in all four places
• The DK24 mark (/Consortium Logo.svg linking to https://dk24.org/) stays hardcoded as permanent attribution and is deliberately not configurable
• Sponsors come from config.sponsorLogos; an empty array means the "In Collaboration With" block does not render at all
• Nitte_logo.webp and Fiza_logo.webp are hardcoded corner overlays repeated across four screens — drive them from config.cornerLogoLeft / cornerLogoRight, with null rendering nothing
• Add docs/BRANDING.md: what to drop into public/, recommended dimensions, and a note that the shipped Nitte / Fiza / COSC / SOSC / Sceptix marks belong to those organizations and should be replaced rather than reused

Keep the logo files in public/ so the example config works out of the box.

Blocked by the ConfigContext issue.