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/views/dropbox.py

@@ -14,6 +14,19 @@ from app.views.base import APIRouter, Depends, get_db, require_login, settings,
 router = APIRouter()
 
 
+def _get_dropbox_callback_url(request: Request) -> str:
+    """Return the Dropbox OAuth callback URL.
+
+    Uses ``PUBLIC_BASE_URL`` when configured so that the redirect URI displayed
+    to the user (and registered in the Dropbox developer console) matches the
+    one used in the OAuth authorization request.  Falls back to deriving the URL
+    from the incoming request when ``PUBLIC_BASE_URL`` is not set.
+    """
+    if settings.public_base_url:
+        return settings.public_base_url.rstrip("/") + "/dropbox-callback"
+    return f"{request.url.scheme}://{request.url.netloc}/dropbox-callback"
+
+
 @router.get("/dropbox-setup")
 @require_login
 async def dropbox_setup_page(
@@ -30,6 +43,8 @@ async def dropbox_setup_page(
     path from the integration's existing config is pre-populated; global
     admin credentials are never exposed in this mode.
     """
+    callback_url = _get_dropbox_callback_url(request)
+
     if integration_id is not None:
         owner_id = get_current_owner_id(request)
         integration = (
@@ -67,6 +82,7 @@ async def dropbox_setup_page(
                     "app_secret_value": "",
                     "refresh_token_value": "",
                     "global_creds_available": global_creds_available,
+                    "callback_url": callback_url,
                 },
             )
 
@@ -86,6 +102,7 @@ async def dropbox_setup_page(
             "integration_id": integration_id,
             "integration_name": None,
             "integration_type": None,
+            "callback_url": callback_url,
         },
     )
 
@@ -116,5 +133,6 @@ async def dropbox_callback(request: Request, code: str = None, error: str = None
             "app_key_value": "",  # The callback will prioritize sessionStorage values
             "app_secret_value": "",  # The callback will prioritize sessionStorage values
             "folder_path": "",  # The callback will prioritize sessionStorage values
+            "callback_url": _get_dropbox_callback_url(request),
         },
     )