test: read static assets as utf-8 so locale/render tests run on Windows

34 test files read static/*.js via read_text()/open() without an encoding,
so Python's cp1252 default on Windows choked on non-Latin1 bytes and errored
at collection (Ubuntu CI defaults to utf-8, so it never saw this). Add
explicit encoding="utf-8" to those reads — purely mechanical, no assertion
changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_issue1096_copy_buttons.py

@@ -3,12 +3,12 @@ import re
 
 
 def _src(name: str) -> str:
-    with open(f"static/{name}") as f:
+    with open(f"static/{name}", encoding="utf-8") as f:
         return f.read()
 
 
 def _py_src() -> str:
-    with open("api/helpers.py") as f:
+    with open("api/helpers.py", encoding="utf-8") as f:
         return f.read()