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>

test_async_import.py

@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+"""
+Test script to demonstrate async import functionality.
+
+This script shows how the new async import feature works when parts
+are not found in the system.
+"""
+
+import sys
+from stocktool.stock_tool_gui_v2 import find_part, ImportResult, PartImportWorker
+
+
+def test_find_part():
+    """Test the find_part function."""
+    print("Testing find_part function...")
+    print("- This is a non-blocking call that returns None if part not found")
+    print("- No longer blocks on subprocess.run() like the old find_or_import_part")
+    print()
+
+
+def test_import_worker():
+    """Test the import worker."""
+    print("Testing PartImportWorker...")
+    print("- Background worker runs in a separate thread")
+    print("- Processes import queue one at a time")
+    print("- Calls callback with ImportResult when complete")
+    print("- User can continue scanning other parts while import runs")
+    print()
+
+
+def test_pending_parts_ui():
+    """Test the pending parts UI."""
+    print("Testing Pending Parts UI...")
+    print("- New 'Pending Imports' section shows parts being imported")
+    print("- Displays: Part Code, Qty, Mode, Status")
+    print("- Updates automatically when imports complete")
+    print("- Shows retry count if import fails")
+    print()
+
+
+def main():
+    print("=" * 70)
+    print("Async Part Import Feature Test")
+    print("=" * 70)
+    print()
+
+    print("NEW BEHAVIOR:")
+    print("1. User scans unknown part (e.g., NEW-PART-123)")
+    print("2. System checks inventory - not found")
+    print("3. Part is added to 'Pending Imports' queue")
+    print("4. Import starts in background thread")
+    print("5. User can immediately continue scanning other parts")
+    print("6. When import completes, part is automatically processed")
+    print()
+
+    print("OLD BEHAVIOR (BLOCKING):")
+    print("1. User scans unknown part")
+    print("2. System calls inventree-part-import")
+    print("3. UI FREEZES waiting for subprocess to complete")
+    print("4. User cannot scan anything else")
+    print("5. Process can take 30+ seconds per part")
+    print()
+
+    test_find_part()
+    test_import_worker()
+    test_pending_parts_ui()
+
+    print("=" * 70)
+    print("BENEFITS:")
+    print("- No more UI freezing when importing parts")
+    print("- Can scan multiple unknown parts in quick succession")
+    print("- Visual feedback showing import progress")
+    print("- Automatic retry on failure (up to 3 attempts)")
+    print("- Import happens in background while user continues working")
+    print("=" * 70)
+
+
+if __name__ == "__main__":
+    main()