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>

docs/ConfigurationGuide.md

@@ -19,6 +19,7 @@ Configuration is primarily done through environment variables specified in a `.e
 | `WORKDIR`              | Working directory for the application.                  | `/workdir`                     |
 | `GOTENBERG_URL`        | Gotenberg PDF processing URL.                           | `http://gotenberg:3000`        |
 | `EXTERNAL_HOSTNAME`    | The external hostname for the application.             | `docuelevate.example.com`      |
+| `PUBLIC_BASE_URL`      | Full public base URL including scheme (e.g., `https://docuelevate.example.com`). When set, overrides auto-detected URLs used for OAuth redirect URIs. **Required when your reverse proxy does not forward `X-Forwarded-Proto` headers.** | *(not set)* |
 | `ALLOW_FILE_DELETE`    | Enable file deletion in the web interface (`true`/`false`). | `true`                      |
 | `COMPLIANCE_ENABLED`   | Enable the compliance templates dashboard (GDPR, HIPAA, SOC 2). | `true`                      |
 | `FACTORY_RESET_ON_STARTUP` | Wipe all user data on every startup (demo/testing). | `false` |