feat(log): add --log-user-agent option to log User-Agent header - #750

Summary

Closes #576.

Adds a --log-user-agent CLI flag (env: SERVER_LOG_USER_AGENT, TOML: log_user_agent) to include the User-Agent request header in access logs.

Changes

Follows the exact same pattern as --log-remote-address, --log-x-real-ip, and --log-forwarded-for:

1. Added log_user_agent: bool field to RequestHandlerOpts
2. Extract User-Agent header in log_addr::pre_process() when enabled
3. Added to the tracing::info! span alongside other address fields

Files changed

• src/handler.rs — added log_user_agent field to RequestHandlerOpts
• src/log_addr.rs — extract and log User-Agent header
• src/settings/cli.rs — added --log-user-agent CLI option
• src/settings/file.rs — added TOML config option
• src/settings/mod.rs — wired config file override
• src/server/opts.rs — wired to handler opts
• src/testing.rs — updated test helper

Usage

static-web-server --log-user-agent
# or
SERVER_LOG_USER_AGENT=true static-web-server

TOML:

[general]
log-user-agent = true