feat: Allow configuring HOME_DIR via environment variable

[yanhao98]

This change allows users to customize the application's home directory by setting the CLAUDE_CODE_ROUTER_DIR environment variable.

• Modified src/constants.ts to use path.resolve with the CLAUDE_CODE_ROUTER_DIR environment variable, falling back to the default ~/.claude-code-router if not set.
• Refactored src/cli.ts, src/server.ts, src/index.ts, src/utils/index.ts, and src/utils/modelSelector.ts to use the centralized HOME_DIR constant, removing hardcoded paths.
• Updated README.md, README_zh.md, and CLAUDE.md to document this new feature and provide generic examples.
• Added a comment to docker-compose.yml to show how to use the environment variable.
• Updated ui/config.example.json to use a generic placeholder path.

[yanhao98]

@copilot open a new pull request to apply changes based on the comments in this thread