fix: protect wb.close() with try/finally, strengthen width assertions, declare openpyxl dev dep #83

- I-1: Add explicit lower bounds to width assertions (>= 15 for autre, >= 40 for objet)
- I-2: Wrap write_excel() and close() in try/finally to ensure cleanup even on exception
- M-2: Add openpyxl to dev dependencies in pyproject.toml

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

pyproject.toml

@@ -35,6 +35,7 @@ dev = [
   "webdriver-manager",
   "dash[testing]",
   "fastexcel",
+  "openpyxl",
 ]
 
 [tool.pytest.ini_options]