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>

frontend/templates/dropbox_callback.html

@@ -108,7 +108,8 @@ document.addEventListener('DOMContentLoaded', function() {
     }
   }
 
-  const redirectUri = window.location.origin + "/dropbox-callback";
+  // Use server-provided callback URL (respects PUBLIC_BASE_URL) with fallback to window.location.origin
+  const redirectUri = {{ callback_url | tojson }} || (window.location.origin + "/dropbox-callback");
 
   // Automatically exchange the code for a refresh token
   if (code) {