feat(integrations): add per-user OAuth wizard with user-mode for Dropbox, OneDrive, Google Drive

- Add user_mode to dropbox/onedrive/google_drive setup views that loads integration config
- Show user-friendly auth wizard when integration_id is provided (user mode)
- In user mode: show integration name, current folder, back-to-integrations link
- In callback templates: only save credentials (not config) for user integrations
- In integrations dashboard: show Authorize/Re-Authorize button for all OAuth types
- Add WATCH_FOLDER OAuth support: detect source_type in config for auth button
- isOAuthType() and oauthLink() now accept full integration object

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/dropbox_callback.html

@@ -145,11 +145,10 @@ document.addEventListener('DOMContentLoaded', function() {
             app_key: appKey,
             app_secret: appSecret,
           };
-          const cfgUpdate = {};
-          if (folderPath) cfgUpdate.folder = folderPath;
 
+          // Only send credentials — the integration's config (folder, source_type, etc.)
+          // is already set and must not be overwritten here.
           const body = { credentials: creds };
-          if (Object.keys(cfgUpdate).length > 0) body.config = cfgUpdate;
 
           document.getElementById('processing-message').innerHTML =
             '<p>Saving credentials to your integration...</p>';