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/page.tsx

@@ -13,7 +13,8 @@ import { AvatarFallbackWithName } from "@/components/avatar-fallback";
 import { revokeShareLinkAction } from "./actions";
 import { getHouseholdApiTokenStatus } from "@/modules/_core/api-token";
 import { ApiTokenSettings } from "@/components/api-token-settings";
-import { AssistantOptIn } from "@/components/assistant-opt-in";
+import { AssistantSettings } from "@/components/assistant-settings";
+import { AGENT_SYSTEM_PROMPT } from "@/modules/agent/tools";
 import { DefaultEventRemindersSetting } from "@/components/default-event-reminders-setting";
 import { listCalendars } from "@/modules/calendar/server/queries";
 import { listLists } from "@/modules/lists/server/queries";
@@ -322,6 +323,8 @@ function AppearanceSection({
     themeCalView: string;
     themeNavStyle: string;
     assistantEnabled: boolean;
+    assistantName: string;
+    assistantSystemPrompt: string | null;
   };
 }) {
   return (
@@ -355,7 +358,12 @@ function AppearanceSection({
           <MessageCircle className="size-4 text-[var(--ink-mute)]" />
         </CardHeader>
         <CardContent>
-          <AssistantOptIn enabled={user.assistantEnabled} />
+          <AssistantSettings
+            enabled={user.assistantEnabled}
+            name={user.assistantName}
+            systemPrompt={user.assistantSystemPrompt}
+            defaultSystemPrompt={AGENT_SYSTEM_PROMPT}
+          />
         </CardContent>
       </Card>
     </>