feat: update overriding mimetype

```console
$ fava bcs/main.bc -p 5001  # <- python 3.12
Invalid mimetype set for '.js', overriding
Starting Fava on http://127.0.0.1:5001
```

- `mimetype` sometimes broken on windows: <python/cpython#97646>
- use `text/javascript`: <beancount#1446>

src/fava/application.py

@@ -93,11 +93,11 @@
 ]
 
 
-if mimetypes.types_map.get(".js") != "application/javascript":
+if not mimetypes.types_map.get(".js", "").endswith("/javascript"):
     # This is sometimes broken on windows, see
     # https://github.com/beancount/fava/issues/1446
     logging.error("Invalid mimetype set for '.js', overriding")
-    mimetypes.add_type("application/javascript", ".js")
+    mimetypes.add_type("text/javascript", ".js")
 
 
 def _ledger_slugs_dict(ledgers: Iterable[FavaLedger]) -> dict[str, FavaLedger]: