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/settings/assistant-actions.ts

@@ -2,13 +2,58 @@
 
 import { eq } from "drizzle-orm";
 import { revalidatePath } from "next/cache";
+import { z } from "zod";
 import { db } from "@/lib/db";
+import {
+  DEFAULT_ASSISTANT_NAME,
+  MAX_ASSISTANT_NAME_LENGTH,
+  MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH,
+} from "@/lib/assistant-config";
 import { users } from "@/modules/_core/schema";
 import { getCurrentSession } from "@/lib/session";
 
+const assistantNameSchema = z
+  .string()
+  .trim()
+  .min(1, "Name is required")
+  .max(MAX_ASSISTANT_NAME_LENGTH);
+
+const assistantSystemPromptSchema = z
+  .string()
+  .trim()
+  .min(1, "Prompt cannot be empty")
+  .max(MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH);
+
+function revalidateAssistantSurfaces() {
+  revalidatePath("/settings");
+  revalidatePath("/", "layout");
+}
+
 export async function setAssistantEnabled(enabled: boolean): Promise<void> {
   const { user } = await getCurrentSession();
   await db.update(users).set({ assistantEnabled: enabled }).where(eq(users.id, user.id));
-  revalidatePath("/settings");
-  revalidatePath("/", "layout");
+  revalidateAssistantSurfaces();
+}
+
+export async function setAssistantName(name: string): Promise<void> {
+  const parsed = assistantNameSchema.parse(name);
+  const { user } = await getCurrentSession();
+  await db.update(users).set({ assistantName: parsed }).where(eq(users.id, user.id));
+  revalidateAssistantSurfaces();
+}
+
+export async function setAssistantSystemPrompt(prompt: string | null): Promise<void> {
+  const { user } = await getCurrentSession();
+  const normalized = prompt === null ? null : assistantSystemPromptSchema.parse(prompt);
+
+  await db.update(users).set({ assistantSystemPrompt: normalized }).where(eq(users.id, user.id));
+  revalidateAssistantSurfaces();
+}
+
+export async function resetAssistantName(): Promise<void> {
+  await setAssistantName(DEFAULT_ASSISTANT_NAME);
+}
+
+export async function resetAssistantSystemPrompt(): Promise<void> {
+  await setAssistantSystemPrompt(null);
 }