feat: Add video library with upload/browse/delete UI

[getter111]

Description

This PR adds video library UI components, enabling video upload, browsing, and delete functionality.

Key Changes

• VideoLibrary Component: Grid/list view browser with search, filtering, and delete functionality
• VideoUpload Component: Drag-and-drop upload interface with progress tracking and file validation (video types, 8GB limit)
• VideoLibraryModal Component: Full-screen modal overlay that displays VideoLibrary in a dialog
• API Client (videoUpload.ts): Proxies all video operations through backend FastAPI server (no S3 credentials exposed to frontend)
• ChatbarSettings Integration: Add VideoLibraryModel component in the sidebar to open video library modal

Related PRs

This PR depends on NVIDIA/NeMo-Agent-Toolkit#1195

Closes

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[willkill07]

@getter111 please see the PR review under NAT for feedback. Some changes would have to be incorporated here.