fix(dropbox): fix Invalid redirect_uri error by adding PUBLIC_BASE_URL config and URL-encoding

- Add PUBLIC_BASE_URL optional config to override auto-detected OAuth redirect URIs
  when behind a reverse proxy that doesn't forward X-Forwarded-Proto headers
- Add _build_dropbox_redirect_uri() helper in app/api/dropbox.py
- URL-encode redirect_uri in server-side Dropbox authorization URL
- Add _get_dropbox_callback_url() helper in app/views/dropbox.py
- Pass callback_url to both setup and callback templates
- Update templates to use server-provided callback_url instead of window.location.origin
- Update settings_service.py to register new setting
- Update .env.demo, ConfigurationGuide.md, and DropboxSetup.md documentation
- Add tests for new helper functions and global-authorize-url endpoint

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -3,6 +3,7 @@ WORKDIR=/workdir
 DATABASE_URL=sqlite:///./app/database.db
 REDIS_URL=redis://redis:6379/0
 EXTERNAL_HOSTNAME=docuelevate.example.com
+# PUBLIC_BASE_URL=https://docuelevate.example.com  # Full URL with scheme; required when X-Forwarded-Proto is not forwarded by your proxy
 GOTENBERG_URL=http://gotenberg:3000
 ALLOW_FILE_DELETE=true  # Allow deletion of file records
 COMPLIANCE_ENABLED=true  # Enable compliance templates dashboard (GDPR, HIPAA, SOC 2)