Add In-App Document Viewer

[AliiiBenn]

Description

Currently, attachments (PDFs, images) are stored but can't be viewed in the app. Users must download and open externally.

Requirements

1. Image Viewer:

   • View images (PNG, JPG) inline
   • Lightbox/popup for full-size view

2. PDF Viewer:

   • View PDFs inline in the app
   • Use browser's embed or a library like react-pdf

3. Integration Points:

   • Click attachment in any table to view
   • Documents page should show previews
   • Employee detail should show document thumbnails

Current State

The attachments table exists and files are stored, but no viewing capability.

Acceptance Criteria

• Can view images without downloading
• Can view PDFs in app
• Works from Documents page
• Works from Employee detail (issue Add Employee Detail Page #95)