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/lib/assistant-config.ts

@@ -0,0 +1,10 @@
+import { AGENT_SYSTEM_PROMPT } from "@/modules/agent/tools";
+
+export const DEFAULT_ASSISTANT_NAME = "Assistant";
+export const MAX_ASSISTANT_NAME_LENGTH = 40;
+export const MAX_ASSISTANT_SYSTEM_PROMPT_LENGTH = 8000;
+
+export function resolveAssistantSystemPrompt(customPrompt: string | null | undefined): string {
+  const trimmed = customPrompt?.trim();
+  return trimmed ? trimmed : AGENT_SYSTEM_PROMPT;
+}