[FE-55] Implement cookie consent banner with preference management

[mftee]

Overview

Build a cookie consent banner that appears on first visit and lets users accept or decline non-essential cookies, persisting their choice. All work goes in frontend/sandbox/.

Acceptance Criteria

• Component at frontend/sandbox/components/CookieConsentBanner.tsx
• Appears at the bottom of the screen on first visit (no prior consent stored)
• Two buttons: Accept All and Manage Preferences
• Manage Preferences opens a modal with toggles for: Analytics cookies, Marketing cookies (Essential cookies always on and non-toggleable)
• Consent choice stored in localStorage under cookie_consent
• Banner does not reappear once a choice is made
• Cookie Settings link in the footer re-opens the preferences modal
• Demo page at frontend/sandbox/page.tsx includes a Reset consent button for testing