Add Python REPL widget for WebGL build

Features:
- JSFiddle-style code editor panel alongside game canvas
- Run button (or Ctrl+Enter) executes Python code
- Reset button reloads game.py
- Tab key inserts 4 spaces for proper indentation
- Shows >>> prompt with code preview
- Displays repr of last expression (like Python REPL)
- Error highlighting in red, success in green
- Canvas focus handling with visual indicator

C++ exports (callable from JavaScript):
- run_python_string(code) - simple execution
- run_python_string_with_output(code) - captures stdout/stderr + expr repr
- reset_python_environment() - reloads game.py

JavaScript API available in console:
- runPython(code) - execute Python and get output
- resetGame() - reset to initial state
- FS.readFile/writeFile - access virtual filesystem

Also fixes canvas focus issues on page load.

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

CMakeLists.txt

@@ -255,7 +255,8 @@ if(EMSCRIPTEN)
         -sUSE_SQLITE3=1
         -sALLOW_MEMORY_GROWTH=1
         -sSTACK_SIZE=2097152
-        -sEXPORTED_RUNTIME_METHODS=ccall,cwrap
+        -sEXPORTED_RUNTIME_METHODS=ccall,cwrap,FS
+        -sEXPORTED_FUNCTIONS=_main,_run_python_string,_run_python_string_with_output,_reset_python_environment
         -sASSERTIONS=2
         -sSTACK_OVERFLOW_CHECK=2
         -fexceptions