docs: full README for assignment showcase

docs: trim README to showcase Quick Start

docs: remove duplicate 'Problems Encountered & Solutions' section

docs: expand README Troubleshooting & Fixes

ui: force input text color to black (title + url)

ui: set input text color to black in add bookmark form

ui: onboarding tooltip + subtle hover lift on bookmark cards

feat: undo snackbar for deletes (reinsert on undo)

ui: improved bookmark cards (favicon, domain excerpt, quick actions)

fix: reliable cross-tab sync using localStorage fallback for inactive/split tabs

feat: broadcast bookmark deletions to other tabs for immediate sync

feat: cross-tab sync (BroadcastChannel) for immediate bookmark updates

bookmarks: optimistic delete + realtime payload debug log

Save local edits: callback fix, deploy guide, supabase client, security notes

Fix OAuth callback to detect session from URL hash