fix(desktop): hide model selector in Tauri runtime — SaaS token pool routes models

Model selector was cosmetic-only in desktop mode: chatStream never passes
model param to backend. Hiding prevents user confusion and 404 errors when
selecting models not in SaaS token pool.

Also adds E2E test report covering 168 messages, 4 bugs found (P0 fixed).

desktop/src/components/ChatArea.tsx

@@ -31,6 +31,7 @@ import { ReasoningBlock } from './ai/ReasoningBlock';
 import { StreamingText } from './ai/StreamingText';
 import { ChatMode } from './ai/ChatMode';
 import { ModelSelector } from './ai/ModelSelector';
+import { isTauriRuntime } from '../lib/tauri-gateway';
 import { SuggestionChips } from './ai/SuggestionChips';
 import { PipelineResultPreview } from './pipeline/PipelineResultPreview';
 import { PresentationContainer } from './presentation/PresentationContainer';
@@ -540,12 +541,14 @@ export function ChatArea({ compact, onOpenDetail }: { compact?: boolean; onOpenD
                 }
               </div>
               <div className="flex items-center gap-2">
-                <ModelSelector
-                  models={models.map(m => ({ id: m.id, name: m.name, provider: m.provider }))}
-                  currentModel={currentModel}
-                  onSelect={setCurrentModel}
-                  disabled={isStreaming}
-                />
+                {!isTauriRuntime() && (
+                  <ModelSelector
+                    models={models.map(m => ({ id: m.id, name: m.name, provider: m.provider }))}
+                    currentModel={currentModel}
+                    onSelect={setCurrentModel}
+                    disabled={isStreaming}
+                  />
+                )}
                 <Button
                   variant="primary"
                   size="sm"