feat: Add async background part import queue

Implements non-blocking part import to prevent UI freezing when scanning unknown parts.

Features:
- Background import worker thread processes unknown parts
- New "Pending Imports" UI section shows import progress
- User can continue scanning other parts while imports run
- Automatic retry on failure (up to 3 attempts)
- Parts automatically processed when import completes

Changes:
- Added PendingPart and ImportResult data structures
- Added PartImportWorker background thread class
- Replaced blocking find_or_import_part() with async find_part()
- Added pending parts queue UI with status display
- Added _on_import_complete() callback handler
- Added _update_pending_parts_ui() for real-time updates
- Added proper cleanup on window close

Benefits:
- No more 30+ second UI freezes during part imports
- Can scan multiple unknown parts in quick succession
- Visual feedback showing import status for each part
- Automatic error handling and retry logic

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -8,6 +8,7 @@ A comprehensive barcode scanning application for InvenTree inventory management.
 - **Stock Updates**: Update existing stock levels
 - **Stock Checking**: Check current stock levels
 - **Part Location**: Find where parts are stored
+- **Async Part Import**: Non-blocking background import for unknown parts
 - **Server Connection Monitoring**: Real-time connection status
 - **Barcode Command Support**: Control the app via barcode commands
 
@@ -61,6 +62,22 @@ token: your-api-token-here
 2. **Select Mode**: Choose operation mode (Add Stock, Update Stock, Check Stock, or Locate Part)
 3. **Scan Parts**: Scan part barcodes to perform operations
 
+### Async Part Import (New!)
+
+When you scan a part that doesn't exist in the system:
+
+1. **Part is automatically queued** for background import
+2. **Continue scanning other parts** - no waiting required!
+3. **Watch the "Pending Imports" section** to see import progress
+4. **Part is automatically processed** when import completes
+5. **Failed imports retry automatically** (up to 3 attempts)
+
+**Benefits**:
+- No more UI freezing when importing parts
+- Scan multiple unknown parts in quick succession
+- Visual feedback showing import progress
+- Automatic error handling and retry logic
+
 ### Barcode Commands
 
 The application supports special barcode commands for quick mode switching: