feat(odin): scope to invoice processing only (guard general chat)

Odin currently advertises general Q&A via the /chat path, which burns API
credits on open-ended questions. Guard it: a text-only message makes NO AI
call — it gets a canned reply telling the user to attach an invoice. The
invoice path (attachments → extract) is unchanged. Removing the guard
re-enables the general /chat path for a later phase.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components/odin/OdinChat.tsx

@@ -208,6 +208,24 @@ export default function OdinChat() {
     const files = attachments.map((a) => a.file);
     if (!text && files.length === 0) return;
 
+    // Guard: Odin is scoped to invoice processing only (for now). A text-only
+    // message makes NO AI call — it gets a canned reply locally, so open-ended
+    // chat can't burn API credits. The invoice path (attachments) runs normally.
+    // (Removing this guard re-enables the general /chat path below for Phase 2.)
+    if (files.length === 0) {
+      setTurns((t) => [
+        ...t,
+        { role: "user", content: text },
+        {
+          role: "assistant",
+          content:
+            "Momentálně umím zpracovat pouze přijaté faktury. Přiložte prosím fakturu (PDF) a já z ní načtu údaje k uložení.",
+        },
+      ]);
+      setInput("");
+      return;
+    }
+
     setBusy(true);
     const prevTurns = turns;