fix: 100% coverage, WCAG accessibility fixes, and documentation updates for per-user OAuth wizards

Test coverage:
- Add config=None branch test for dropbox, onedrive, google_drive views (100% coverage)
- Add WATCH_FOLDER source-type test (folder_path vs folder key)
- Add integration-not-found fallback-to-admin-mode test

WCAG 2.1 AA fixes:
- Add aria-labelledby="modalTitle" to role="dialog" modals in setup templates
- Add aria-hidden="true" to decorative SVGs in callback templates
- Add role="status" aria-label="Loading" to spinner divs
- Add aria-live="polite" to processing-message and success/folder-selection regions
- Add role="alert" aria-live="assertive" to error containers
- Update "Return to Setup" link to preserve integration_id in user mode

Docs: update DropboxSetup.md, GoogleDriveSetup.md, OneDriveSetup.md with per-user OAuth flow section

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

frontend/templates/google_drive.html

@@ -404,7 +404,7 @@ GOOGLE_DRIVE_FOLDER_ID={{ folder_id|default('YOUR_FOLDER_ID', true) }}
   </div>
 
   <!-- Result Modal -->
-  <div id="resultModal" class="fixed inset-0 bg-gray-600 bg-opacity-50 hidden overflow-y-auto h-full w-full z-50" aria-modal="true" role="dialog">
+  <div id="resultModal" class="fixed inset-0 bg-gray-600 bg-opacity-50 hidden overflow-y-auto h-full w-full z-50" aria-modal="true" role="dialog" aria-labelledby="modalTitle">
     <div class="relative top-20 mx-auto p-5 border w-11/12 md:w-1/2 lg:w-1/3 shadow-lg rounded-md bg-white">
       <div class="mt-3 text-center">
         <div id="modalIcon" class="mx-auto flex items-center justify-center h-12 w-12 rounded-full bg-green-100 mb-4">