[FIX] Add error feedback and toast notification to CategoryFormModal

[Siddh2024]

Description

Added user-facing error feedback to the category creation modal. Previously, API errors were silently swallowed with only a console.error call.

Changes Made

• Added submitError state variable to track API error messages
• Added inline error banner with AlertCircle icon that appears on failure
• Added toast.success() on successful category creation
• Added toast.error() with the API error message on failure
• Error clears automatically when the user edits any field
• Imported AlertCircle from lucide-react and toast from react-hot-toast

Testing

1. Open Add Category modal
2. Submit with valid data → toast.success shown, modal closes
3. Force API error (e.g., network disconnect) → inline error banner + toast.error shown
4. Edit a field → error clears automatically

Fixes #283