Migrate ExcelConverter from Python to TypeScript

Replace pandas+openpyxl with exceljs for Excel file processing. This
migration enables seamless integration with the Electron main process
and maintains 1:1 functional parity with the Python implementation.

Key changes:
- Add exceljs dependency (v4.4.0)
- Implement ExcelConverter class in TypeScript with strict types
- Add comprehensive unit tests (13 test cases, all passing)
- Support field name mapping, order parsing, and material extraction
- Handle empty data tables and file conflicts gracefully

Verification:
- Successfully tested with sample Excel file (99 orders, 625 records)
- All unit tests passing
- Output format matches Python version

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -25,6 +25,7 @@
     "@electron-toolkit/preload": "^3.0.2",
     "@electron-toolkit/utils": "^4.0.0",
     "dotenv": "^17.3.1",
+    "exceljs": "^4.4.0",
     "playwright-core": "^1.58.2"
   },
   "devDependencies": {