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/utils/settings_service.py

@@ -99,6 +99,18 @@ SETTING_METADATA = {
         "required": True,  # Required for OAuth redirects and external URLs
         "restart_required": True,
     },
+    "public_base_url": {
+        "category": "Core",
+        "description": (
+            "Full public base URL including scheme (e.g., https://docuelevate.example.com). "
+            "When set, overrides auto-detected URLs for OAuth redirect URIs. "
+            "Required when behind a reverse proxy that does not forward X-Forwarded-Proto."
+        ),
+        "type": "string",
+        "sensitive": False,
+        "required": False,
+        "restart_required": True,
+    },
     "debug": {
         "category": "Core",
         "description": "Enable debug mode for verbose logging",