Add Scan Receipt button to create transactions from receipt photos

- New backend endpoint POST /transactions/parse-receipt (file upload, no existing txn needed)
- Refactored AI call logic into shared _call_ai_parse helper (no duplication)
- Scan Receipt button in transactions toolbar → file picker → AI parse → pre-filled form
- TransactionFormModal accepts initialValues prop to pre-populate fields from receipt
- "Fields pre-filled from receipt" banner shown in form when AI-populated
- Scan error displayed inline with dismiss button
- Supports JPEG, PNG, WebP, PDF (Anthropic) or images (OpenAI)

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

frontend/src/api/settings.ts

@@ -41,3 +41,12 @@ export async function parseReceipt(txnId: string, attachmentId: string): Promise
   const { data } = await api.post(`/transactions/${txnId}/attachments/${attachmentId}/parse`);
   return data;
 }
+
+export async function parseReceiptFile(file: File): Promise<ParsedReceipt> {
+  const form = new FormData();
+  form.append("file", file);
+  const { data } = await api.post("/transactions/parse-receipt", form, {
+    headers: { "Content-Type": "multipart/form-data" },
+  });
+  return data;
+}