feat: per-user assistant name and system prompt customization

Each user can rename the AI assistant and edit their own system prompt in Settings.

src/app/api/agent/chat/route.ts

@@ -1,7 +1,7 @@
 import { z } from "zod";
 import { apiError, apiJson } from "@/lib/api-handler";
 import { resolveApiAuth } from "@/lib/api-auth";
-import { getAssistantEnabled } from "@/lib/assistant-preference";
+import { getAssistantPreferences, resolveAssistantSystemPrompt } from "@/lib/assistant-preference";
 import { isLlmConfigured } from "@/lib/llm";
 import { encodeSseEvent } from "@/modules/agent/server/progress";
 import { runAgentChat } from "@/modules/agent/server/run";
@@ -25,11 +25,13 @@ export async function POST(request: Request) {
     return apiError("Unauthorized", 401);
   }
 
-  const assistantEnabled = await getAssistantEnabled(auth.userId);
-  if (!assistantEnabled) {
+  const assistant = await getAssistantPreferences(auth.userId);
+  if (!assistant.enabled) {
     return apiError("Assistant not enabled", 403);
   }
 
+  const systemPrompt = resolveAssistantSystemPrompt(assistant.systemPrompt);
+
   let body: unknown;
   try {
     body = await request.json();
@@ -54,6 +56,7 @@ export async function POST(request: Request) {
           const result = await runAgentChat({
             messages: parsed.data.messages,
             request,
+            systemPrompt,
             onProgress: send,
           });
 
@@ -87,6 +90,7 @@ export async function POST(request: Request) {
     const result = await runAgentChat({
       messages: parsed.data.messages,
       request,
+      systemPrompt,
     });
 
     return apiJson({