[FEAT] Add `Navbar` and `Footer` Components

[Emmzyemms]

Description:
Navbar and Footer are imported on public pages (/, /login, /onboarding). Their completeness needs to be verified.

Acceptance Criteria:

• Navbar: logo, "Sign In" CTA (if unauthenticated), user avatar dropdown (if authenticated), mobile hamburger menu
• Footer: logo, links to Privacy, Terms, Cookies, and GitHub; copyright
• Both components are fully responsive (mobile-first)
• Storybook stories for Navbar: unauthenticated, authenticated, mobile
• Storybook story for Footer

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[Ayom04]

I will like to work on this issue.