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/google_drive.html

@@ -358,6 +358,12 @@ GOOGLE_DRIVE_FOLDER_ID={{ folder_id|default('YOUR_FOLDER_ID', true) }}
 {% 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);
+  }
+
   // Tab elements
   const oauthTabBtn = document.getElementById('oauth-tab-btn');
   const saTabBtn = document.getElementById('sa-tab-btn');