Refactor configuration loading with dotenv and pydantic-settings

[74th]

Summary

This PR modernizes configuration handling across the Python server and example apps by introducing environment-based settings with python-dotenv and pydantic-settings.

What changed

• Added .env.template and updated setup docs for environment variable configuration.
• Refactored speech recognition/synthesis configuration paths to use pydantic-settings.
• Updated multiple example apps to load environment variables via dotenv.
• Improved and expanded Japanese documentation (setup_ja, server_ja, run_sample_app_ja, firmware_ja).
• Added documentation notes for Claude Agent SDK file system permissions and marked related sections as WIP.

Impact

• Configuration is now more explicit and consistent across modules.
• Local setup should be easier and less error-prone with standardized .env-based settings.

Notes

• This branch currently contains 23 changed files and includes both implementation and documentation updates.