Build multi-page dashboard layout with sidebar navigation

[Kr1491]

🖥️ Current Behavior

The app is a single page with no navigation. Everything — upload, status, and results — is cramped into one page with no way to navigate between different sections.

✅ Expected Behavior

A full dashboard layout with:

• Persistent sidebar on the left with navigation links
• Active page highlighted in the sidebar
• Header showing the system name
• Main content area that changes based on selected page

📄 Pages to Include

• Dashboard — overview and stats
• Upload — drag and drop file upload
• Processing — live queue monitor
• Validation — review uncertain documents
• Documents — full document library
• Export — download classification results

🔧 Implementation

• Convert index.html into a base layout template
• Create separate HTML templates for each page
• Use Flask routes for each page (/dashboard, /upload,
  /processing, /validation, /documents, /export)
• Highlight active sidebar link using Flask's request.path

📌 Note

This is a prerequisite for all other page-specific issues.
Issues 3 through 8 depend on this being completed first.

🏷️ Labels

enhancement frontend ui