IAV-577/feat(agent-custom-instructions): Frontend integration for Agent Custom Instructions management

Summary

Implement frontend integration for managing Agent Custom Instructions grouped by category (Suggestions and Restrictions).

Changes

• Add Zustand store for agent custom instructions state management
• Implement CRUD operations (create, update, delete) for instructions
• Add activate/deactivate toggle functionality
• Add delete confirmation dialog
• Implement service API layer for backend communication

Features

• Display instructions grouped by category (Suggestions / Restrictions)
• Create new instructions:
  • Suggestions: with optional document attachment
  • Restrictions: title and content only
• Edit existing instructions
• Activate/Deactivate instructions via toggle
• Delete instructions with confirmation dialog

Related Issues

Closes IAV-577