Add: Complete inventory sync following SteamAuth pattern

Changes:
1. Added inventorySync.js to manifest.json content scripts
2. Updated inventorySync to use Chrome message passing (not window.postMessage)
3. Added background handler for 'syncInventory' action
4. Background forwards inventory to /api/inventory/sync endpoint

Architecture (following SteamAuth):
- Content script (inventorySync.js) fetches inventory from Steam API
- Sends to background via chrome.runtime.sendMessage
- Background forwards to backend API with credentials
- Returns result to content script

Benefits:
- Bypasses 10-day restriction (user authenticated to own account)
- No external dependencies (no SteamAuth needed)
- Follows Chrome extension best practices
- Clean separation: content → background → backend

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

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

Author: rainerstudios

background.js

@@ -1058,6 +1058,54 @@ chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
     return true; // Keep message channel open
   }
 
+  // Handle inventory sync (SteamAuth pattern)
+  if (request.action === 'syncInventory') {
+    (async () => {
+      try {
+        console.log('[Background] 📦 Inventory sync request received');
+        console.log('[Background] 📦 Inventory contains:', request.payload.inventory.length, 'items');
+
+        const response = await fetch(
+          `${API_BASE_URL}/api/inventory/sync`,
+          {
+            method: 'POST',
+            headers: {
+              'Content-Type': 'application/json',
+              'X-Extension-Token': EXTENSION_SECRET,
+            },
+            credentials: 'include',
+            body: JSON.stringify(request.payload),
+          }
+        );
+
+        if (!response.ok) {
+          const errorText = await response.text();
+          let errorData;
+          try {
+            errorData = JSON.parse(errorText);
+          } catch {
+            errorData = { message: errorText };
+          }
+
+          console.error('[Background] ❌ Inventory sync failed:', errorData);
+          sendResponse({
+            success: false,
+            error: errorData.message || `Backend returned ${response.status}`
+          });
+          return;
+        }
+
+        const data = await response.json();
+        console.log('[Background] ✅ Inventory sync successful:', data);
+        sendResponse(data);
+      } catch (error) {
+        console.error('[Background] ❌ Inventory sync error:', error);
+        sendResponse({ success: false, error: error.message });
+      }
+    })();
+    return true; // Keep message channel open
+  }
+
   /**
    * Helper function to fetch transaction import directly (without content script)
    * May fail if session cookies aren't accessible

manifest.json

@@ -45,7 +45,7 @@
         "https://steamcommunity.com/id/*/tradehistory",
         "https://steamcommunity.com/profiles/*/tradehistory"
       ],
-      "js": ["lib/marketIntelligence.js", "lib/inventoryEnhancer.js", "src/infiniteScroll.js", "src/buff163Integration.js", "src/item3DPreview.js", "src/tradeProtectionDisplay.js", "src/floatRarityDisplay.js", "src/multiMarketPricing.js", "src/watchlistButton.js", "src/transactionMonitor.js", "content.js"],
+      "js": ["lib/marketIntelligence.js", "lib/inventoryEnhancer.js", "src/infiniteScroll.js", "src/buff163Integration.js", "src/item3DPreview.js", "src/tradeProtectionDisplay.js", "src/floatRarityDisplay.js", "src/multiMarketPricing.js", "src/watchlistButton.js", "src/transactionMonitor.js", "src/inventorySync.js", "content.js"],
       "css": ["styles/main.css", "styles/inventory.css"],
       "run_at": "document_idle"
     },

src/inventorySync.js

@@ -177,10 +177,7 @@
         },
 
         async sendToBackend(inventory, steamId) {
-            this.log('📤 Sending inventory to backend...');
-
-            // Get backend URL from storage or use default
-            const backendUrl = await this.getBackendUrl();
+            this.log('📤 Sending inventory to backend via background script...');
 
             const payload = {
                 steamId: steamId,
@@ -190,24 +187,23 @@
                 contextId: 2
             };
 
-            this.log(`📤 Sending ${inventory.length} items to ${backendUrl}`);
-
-            const response = await fetch(`${backendUrl}/api/inventory/sync`, {
-                method: 'POST',
-                headers: {
-                    'Content-Type': 'application/json'
-                },
-                body: JSON.stringify(payload),
-                credentials: 'include' // Include session cookies
+            this.log(`📤 Sending ${inventory.length} items to background script`);
+
+            // Send to background script using Chrome message passing (SteamAuth pattern)
+            return new Promise((resolve, reject) => {
+                chrome.runtime.sendMessage({
+                    action: 'syncInventory',
+                    payload: payload
+                }, (response) => {
+                    if (chrome.runtime.lastError) {
+                        reject(new Error(chrome.runtime.lastError.message));
+                    } else if (response && response.error) {
+                        reject(new Error(response.error));
+                    } else {
+                        resolve(response);
+                    }
+                });
             });
-
-            if (!response.ok) {
-                const errorText = await response.text();
-                throw new Error(`Backend returned ${response.status}: ${errorText}`);
-            }
-
-            const result = await response.json();
-            return result;
         },
 
         async getBackendUrl() {