Apply second code review fixes: robustness and correctness

- AiAssistantService: 90s HTTP timeout; RefineWithCorrections uses 29.99 example
  price and explicit "Never return 0" instruction; treat returned 0 as missing and
  keep original price instead
- EbayAuthService: shared static HttpClient for all auth/token calls; SemaphoreSlim
  double-checked locking on GetAppTokenAsync to prevent concurrent token fetches
- EbayPriceResearchService: 15s timeout; require ≥5 samples before suggesting;
  trim top/bottom 10% outliers; BEST_MATCH sort avoids spam/broken listing bias
- PhotoAnalysisView: spinner-show inside try (Issue 1); decimal→double casts use
  Math.Round to avoid 19.99→19.989... drift (Issue 6); Dispatcher.CheckAccess guard
  for off-thread callers (Issue 7); save toast always restarts instead of silently
  dropping rapid saves, eliminating any stuck-flag path (Issue 8)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

EbayListingTool/Services/AiAssistantService.cs

@@ -16,7 +16,7 @@ public class AiAssistantService
 {
     private readonly string _apiKey;
     private readonly string _model;
-    private static readonly HttpClient _http = new();
+    private static readonly HttpClient _http = new() { Timeout = TimeSpan.FromSeconds(90) };
 
     private const string ApiUrl = "https://openrouter.ai/api/v1/chat/completions";
 
@@ -142,20 +142,25 @@ public class AiAssistantService
     public async Task<(string Title, string Description, decimal Price, string PriceReasoning)>
         RefineWithCorrectionsAsync(string title, string description, decimal price, string corrections)
     {
+        var priceContext = price > 0
+            ? $"Current price: £{price:F2}\n\n"
+            : "";
+
         var prompt =
             "You are an expert eBay UK seller. I have a listing with incorrect details that need fixing.\n\n" +
             $"Current title: {title}\n" +
             $"Current description:\n{description}\n\n" +
-            $"Current price: £{price:F2}\n\n" +
+            priceContext +
             $"CORRECTIONS FROM SELLER: {corrections}\n\n" +
             "Rewrite the listing incorporating these corrections. " +
             "Return ONLY valid JSON — no markdown, no explanation:\n" +
             "{\n" +
             "  \"title\": \"corrected eBay UK title, max 80 chars, keyword-rich\",\n" +
             "  \"description\": \"corrected full plain-text eBay UK description\",\n" +
-            "  \"price_suggested\": 0.00,\n" +
+            "  \"price_suggested\": 29.99,\n" +
             "  \"price_reasoning\": \"one sentence why this price\"\n" +
-            "}";
+            "}\n\n" +
+            "price_suggested MUST be a realistic GBP amount greater than 0. Never return 0.";
 
         var json = await CallAsync(prompt);
 
@@ -169,10 +174,11 @@ public class AiAssistantService
                 obj = JObject.Parse(m.Success ? m.Groups[1].Value : json.Trim());
             }
 
+            var parsedPrice = obj["price_suggested"]?.Value<decimal>() ?? 0;
             return (
-                obj["title"]?.ToString()          ?? title,
-                obj["description"]?.ToString()    ?? description,
-                obj["price_suggested"]?.Value<decimal>() ?? price,
+                obj["title"]?.ToString()       ?? title,
+                obj["description"]?.ToString() ?? description,
+                parsedPrice > 0 ? parsedPrice : price, // treat 0 as "not provided", keep original
                 obj["price_reasoning"]?.ToString() ?? ""
             );
         }