feat: add internal AI chat assistant and AI Input Agent page

- Add Gemini-powered internal staff chatbot (free-text input in the
  upper-right bot panel): type "check MARIA GONZALES" to search patient
  and pre-fill eligibility, or "open claims" to navigate directly
- Add /api/ai/internal-chat endpoint with LangGraph + Google Gemini
  classifier (intent: check_eligibility, find_patient, navigate_*)
- Add Users AI Chat settings section in Settings > Advanced > AI Chat
  to configure a custom system prompt for the internal assistant
- Store internal chat system prompt in existing twilioSettings JSON
  blob (no DB migration needed)
- Add AI Input Agent sidebar entry and placeholder page describing
  planned keyboard-automation typing into Open Dental / Eaglesoft

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/Frontend/src/components/layout/sidebar.tsx

@@ -169,6 +169,11 @@ export function Sidebar() {
         path: "/cloud-storage",
         icon: <Cloud className="h-5 w-5 text-sky-500" />,
       },
+      {
+        name: "AI Input Agent",
+        path: "/ai-input-agent",
+        icon: <Bot className="h-5 w-5 text-violet-500" />,
+      },
       {
         name: "AI Dental Shopping",
         path: "/dental-shopping",