Improve chat navigation, model selection, and streaming

lib/chat/messages.ts

@@ -0,0 +1,15 @@
+import type { UIMessage } from "ai";
+
+type MessagePart = UIMessage["parts"][number];
+type ReasoningPart = Extract<MessagePart, { type: "reasoning" }>;
+
+export function getTextContent(parts: MessagePart[]): string {
+  return parts
+    .filter((p) => p.type === "text")
+    .map((p) => p.text || "")
+    .join("");
+}
+
+export function getReasoningPart(parts: MessagePart[]): ReasoningPart | undefined {
+  return parts.find((p): p is ReasoningPart => p.type === "reasoning");
+}

lib/openrouter.ts

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+export const openRouterModels = [
+  { id: 'anthropic/claude-opus-5', name: 'Claude Opus 5' },
+  { id: 'anthropic/claude-opus-5-fast', name: 'Claude Opus 5 (Fast)' },
+  { id: 'anthropic/claude-sonnet-5', name: 'Claude Sonnet 5' },
+  { id: 'anthropic/claude-fable-5', name: 'Claude Fable 5' },
+  { id: 'openai/gpt-6-astra-pro', name: 'GPT-6 Astra Pro' },
+  { id: 'openai/gpt-5.6-luna', name: 'GPT-5.6 Luna' },
+  { id: 'openai/gpt-5.6-luna-pro', name: 'GPT-5.6 Luna Pro' },
+  { id: 'openai/gpt-5.6-sol', name: 'GPT-5.6 Sol' },
+  { id: 'openai/gpt-5.6-sol-pro', name: 'GPT-5.6 Sol Pro' },
+  { id: 'openai/gpt-5.6-terra', name: 'GPT-5.6 Terra' },
+  { id: 'openai/gpt-5.6-terra-pro', name: 'GPT-5.6 Terra Pro' },
+  { id: 'google/gemini-3.8-flash', name: 'Gemini 3.8 Flash' },
+  { id: 'x-ai/grok-4.6', name: 'Grok 4.6' },
+  { id: 'z-ai/glm-5.3', name: 'GLM 5.3' },
+  { id: 'moonshotai/kimi-k3', name: 'Kimi K3' },
+  { id: 'minimax/minimax-m3', name: 'MiniMax M3' },
+  { id: 'deepseek/deepseek-v4-pro', name: 'DeepSeek V4 Pro 0423' },
+  { id: 'deepseek/deepseek-v4-flash-0731', name: 'DeepSeek V4 Flash 0731' },
+  { id: 'meta/muse-spark-1.1', name: 'Muse Spark 1.1' },
+  { id: 'qwen/qwen3.8-max', name: 'Qwen3.8 Max' },
+] as const;
+
+export type OpenRouterModelId = (typeof openRouterModels)[number]['id'];
+
+export const DEFAULT_OPENROUTER_MODEL_ID: OpenRouterModelId = 'openai/gpt-5.6-luna-pro';
+
+export function isOpenRouterModelId(value: unknown): value is OpenRouterModelId {
+  return typeof value === 'string' && openRouterModels.some((model) => model.id === value);
+}