Remove jQuery from the IndexedDB demo

[oiahoon]

Fixes #370

Summary

• remove the external jQuery 1.8.3 dependency from the IndexedDB demo
• replace its DOM updates, event handlers, and form access with native browser APIs
• preserve add/list/delete/clear behavior and HTML, image, PDF, and fallback blob previews

Validation

• reproduced the initialization failure with the jQuery CDN request blocked
• verified the complete demo offline: required-field handling, add/list/delete/clear, uploaded HTML/image/fallback previews, and same-origin URL image loading
• verified uploaded HTML renders without executing embedded scripts
• node --check indexeddb-api/main.js
• tidy -errors -quiet indexeddb-api/index.html
• git diff --check