feat(auth): save OAuth credentials per-user to UserIntegration records

- Add 'Authorize' button to integration cards for OAuth types without credentials
- Add isOAuthType() helper and update info box in create/edit modal
- Accept integration_id query param in Dropbox, Google Drive, OneDrive setup views
- Store integration_id in sessionStorage on setup pages
- Add per-user flow in OAuth callbacks: PUT credentials to /api/integrations/{id}
- Preserve existing global flow as fallback when no integration_id is present

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/templates/dropbox.html

@@ -216,6 +216,12 @@ DROPBOX_FOLDER={{ folder_path|default('/Documents/Uploads', true) }}</code></pre
 {% block scripts %}
 <script>
 document.addEventListener('DOMContentLoaded', function() {
+  // Store integration_id if provided (for per-user OAuth flow)
+  const integrationId = "{{ integration_id or '' }}";
+  if (integrationId) {
+    sessionStorage.setItem('oauth_integration_id', integrationId);
+  }
+
   // Elements
   const startAuthFlowBtn = document.getElementById('start-auth-flow');
   const testTokenBtn = document.getElementById('test-token');