Switch license from GPL-3.0 to Apache-2.0

- Replace LICENSE with properly filled Apache-2.0 (Mikkeli Matlock 2026)
- Add license field to pyproject.toml
- Add SPDX-License-Identifier headers to all .py source files

font_manager.py

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 """
 Font management system with CJK fallback support.
 Handles matplotlib and Qt font configuration with licensing-safe approach.