fix(openrouter): respect the requesting user's saved model config

generatePostText ignored the userId it received and always read an
arbitrary row via getAnyOpenrouterConfig() (LIMIT 1, no ORDER BY),
so a model chosen and saved in Settings could be shadowed by another
stale config row. Now it prefers the requesting user's own config,
falling back to the most recently updated shared one. Also swap the
retired anthropic/claude-3.5-sonnet default for a live OpenRouter slug.

client/src/components/ai-chat.tsx

@@ -32,7 +32,7 @@ export default function AiChat() {
     },
   ]);
   const [input, setInput] = useState("");
-  const [selectedModel, setSelectedModel] = useState("anthropic/claude-3.5-sonnet");
+  const [selectedModel, setSelectedModel] = useState("anthropic/claude-sonnet-4.5");
   const { toast } = useToast();
 
   // Fetch available models from OpenRouter