Make keyboard shortcuts layout-independent

[aminought]

Summary

• derive letter and writing-system shortcut keys from KeyboardEvent.code so shortcuts follow the physical key across layouts
• preserve matching by KeyboardEvent.key for shortcuts already saved in a layout-dependent form
• document layout-independent sidebar and web-panel shortcuts

Testing

• npx.cmd --no-install eslint src/second_sidebar/controllers/shortcuts.mjs src/second_sidebar/utils/keyboard.mjs
• npx.cmd --no-install prettier --check README.md src/second_sidebar/controllers/shortcuts.mjs src/second_sidebar/utils/keyboard.mjs
• git diff --check origin/master...HEAD
• 11 synthetic keyboard-mapping cases
• installed in the stable Firefox 155.0.1 test profile on Windows; startup verification passed