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>

app/config.py

@@ -193,6 +193,16 @@ class Settings(BaseSettings):
     google_docai_processor_id: Optional[str] = None
     google_docai_location: str = "us"  # Processor location, e.g. "us" or "eu"
     external_hostname: str = "localhost"  # Default to localhost
+    public_base_url: Optional[str] = Field(
+        default=None,
+        description=(
+            "The full public base URL of the application, including scheme "
+            "(e.g., 'https://docuelevate.example.com'). "
+            "When set, this overrides the auto-detected URL for OAuth redirect URIs. "
+            "This is required when the application is behind a reverse proxy that does "
+            "not forward X-Forwarded-Proto headers correctly."
+        ),
+    )
 
     # ---------------------------------------------------------------------------
     # Document Translation Settings