feat(chat): file attachments as chips, CoT closed, AI add-to-inventory

Chat fixes (#1/#6/#7):
- chat-input passes raw File[] up instead of inlining file text
- chat-panel sends files as FileUIPart and renders them with the
  ai-elements Attachments component (no more raw text dumps)
- backend extracts text-like file content for the model in routes/chat
- Chain-of-Thought now defaults to collapsed
- file upload works regardless of whether the input has text

AI add-to-inventory (#2):
- new proposeInventory tool (validates items, streams an approval card)
- system prompt distinguishes stocking inventory vs. billing a patient
- ActionPreviewCard commits inventory via POST /api/inventory

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/ai-chat.ts

@@ -56,7 +56,8 @@ export type ActionPreviewKind =
   | "appointment"
   | "task"
   | "prescription"
-  | "invoice";
+  | "invoice"
+  | "inventory";
 export type ActionPreviewData = {
   token: string;
   kind: ActionPreviewKind;