fix: Convert relative image URLs to full InvenTree URLs in web app

Fixes 404 errors when loading part images in the web interface.

Problem:
- InvenTree API returns relative image paths like /media/part_images/...
- Browser tries to load from Flask app instead of InvenTree server
- Results in 404 errors

Solution:
- Detect relative image URLs (starting with /)
- Prepend InvenTree host URL to make them absolute
- Apply to both image and thumbnail properties

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

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

src/stocktool/web/static/js/app.js

@@ -345,6 +345,15 @@ function stockApp() {
                     // Part found
                     this.currentPart = data.part_info;
                     this.currentParameters = data.parameters;
+
+                    // Fix image URLs - convert relative paths to full InvenTree URLs
+                    if (this.currentPart.image && this.currentPart.image.startsWith('/')) {
+                        this.currentPart.image = this.config.host + this.currentPart.image;
+                    }
+                    if (this.currentPart.thumbnail && this.currentPart.thumbnail.startsWith('/')) {
+                        this.currentPart.thumbnail = this.config.host + this.currentPart.thumbnail;
+                    }
+
                     this.log('success', `✅ Found part: ${partCode}`);
 
                     // Execute operation