refactor: Convert to UV Python project with proper package structure

- Restructured project to use src/stocktool package layout
- Migrated to UV for dependency management
- Added pyproject.toml with all dependencies (sv-ttk, pillow, requests, pyyaml)
- Organized test files into tests/ directory
- Updated .gitignore for UV projects
- Comprehensive README with installation and usage instructions
- Removed old unused files (main.py, stock_tool_gui.py, duplicate copy)
- Added CLI entry point: stock-tool command

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

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

tests/test_duplicate_handling.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""
+Test script to demonstrate the updated _add_stock logic.
+This shows how duplicates are prevented by updating existing stock items.
+"""
+
+def simulate_add_stock(part_id, location_id, quantity, existing_stock_items):
+    """
+    Simulate the add stock workflow.
+
+    Args:
+        part_id: Part ID
+        location_id: Location ID
+        quantity: Quantity to add
+        existing_stock_items: List of existing stock items (for simulation)
+
+    Returns:
+        Tuple of (action, stock_id, old_stock, new_stock)
+    """
+    # Find existing stock item for this part at this location
+    existing = None
+    for item in existing_stock_items:
+        if item['part'] == part_id and item['location'] == location_id:
+            existing = item
+            break
+
+    if existing:
+        # Stock item exists - add to it
+        stock_id = existing['pk']
+        old_stock = existing['quantity']
+        new_stock = old_stock + quantity
+
+        # Update the existing item (simulate)
+        existing['quantity'] = new_stock
+
+        return ('updated', stock_id, old_stock, new_stock)
+    else:
+        # No existing stock - create new item
+        new_item = {
+            'pk': len(existing_stock_items) + 100,  # Simulate new ID
+            'part': part_id,
+            'location': location_id,
+            'quantity': quantity
+        }
+        existing_stock_items.append(new_item)
+
+        return ('created', new_item['pk'], 0, quantity)
+
+
+print("Testing Add Stock Duplicate Prevention\n")
+print("=" * 60)
+
+# Simulate existing stock items
+stock_items = []
+
+print("\nScenario 1: Adding stock when NO existing item exists")
+print("-" * 60)
+action, sid, old, new = simulate_add_stock(part_id=456, location_id=476, quantity=150, existing_stock_items=stock_items)
+print(f"Action: {action}")
+print(f"Stock ID: {sid}")
+print(f"Stock change: {old} -> {new} (+{new - old})")
+print(f"Total stock items: {len(stock_items)}")
+print(f"Expected: Created new stock item PASS" if action == 'created' else "FAIL")
+
+print("\n" + "=" * 60)
+print("\nScenario 2: Adding MORE stock to EXISTING item (should UPDATE, not duplicate)")
+print("-" * 60)
+print(f"Current stock items before scan: {len(stock_items)}")
+action, sid, old, new = simulate_add_stock(part_id=456, location_id=476, quantity=100, existing_stock_items=stock_items)
+print(f"Action: {action}")
+print(f"Stock ID: {sid}")
+print(f"Stock change: {old} -> {new} (+{new - old})")
+print(f"Total stock items: {len(stock_items)}")
+print(f"Expected: Updated existing item, NO duplicate PASS" if action == 'updated' and len(stock_items) == 1 else "FAIL")
+
+print("\n" + "=" * 60)
+print("\nScenario 3: Adding stock AGAIN (should continue to UPDATE)")
+print("-" * 60)
+print(f"Current stock items before scan: {len(stock_items)}")
+action, sid, old, new = simulate_add_stock(part_id=456, location_id=476, quantity=50, existing_stock_items=stock_items)
+print(f"Action: {action}")
+print(f"Stock ID: {sid}")
+print(f"Stock change: {old} -> {new} (+{new - old})")
+print(f"Total stock items: {len(stock_items)}")
+print(f"Expected: Updated existing item, still NO duplicate PASS" if action == 'updated' and len(stock_items) == 1 else "FAIL")
+
+print("\n" + "=" * 60)
+print("\nScenario 4: Adding same part to DIFFERENT location (should create new)")
+print("-" * 60)
+print(f"Current stock items before scan: {len(stock_items)}")
+action, sid, old, new = simulate_add_stock(part_id=456, location_id=999, quantity=75, existing_stock_items=stock_items)
+print(f"Action: {action}")
+print(f"Stock ID: {sid}")
+print(f"Stock change: {old} -> {new} (+{new - old})")
+print(f"Total stock items: {len(stock_items)}")
+print(f"Expected: Created new item for different location PASS" if action == 'created' and len(stock_items) == 2 else "FAIL")
+
+print("\n" + "=" * 60)
+print("\nFinal Summary:")
+print("-" * 60)
+for i, item in enumerate(stock_items, 1):
+    print(f"Stock Item {i}: ID={item['pk']}, Part={item['part']}, Location={item['location']}, Qty={item['quantity']}")
+
+print("\nSUCCESS All scenarios demonstrate proper duplicate prevention!")
+print("   - Same part + same location = UPDATE existing")
+print("   - Same part + different location = CREATE new")