Integrates Dropbox OAuth for authentication, adds API routes for authorization, token exchange, and fetching gallery images, and updates the gallery API to support fetching images directly from Dropbox. Replit-Commit-Author: Agent Replit-Commit-Session-Id: 517dfa7b-26ac-463d-a6e1-a58c6df97188 Replit-Commit-Checkpoint-Type: full_checkpoint Replit-Commit-Event-Id: e2398e24-3175-4cbf-91ff-003984c8d042 Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/f209e72a-0939-48fa-84fc-57854de71967/517dfa7b-26ac-463d-a6e1-a58c6df97188/ls5p9ni Replit-Helium-Checkpoint-Created: true