feat: add validated timezone and language env setters

[ZaneL1u]

Summary

• add cac env set timezone|tz with IANA timezone validation and auto detection from proxy exit IP
• add cac env set language|lang with locale/tag normalization to UTF-8 LANG values and auto detection
• update help text, README, env command docs, and changelog entries to document the new behavior

Details

• factor proxy geo lookup into a shared helper so environment creation and later resync use the same detection path
• validate timezone names with Python zoneinfo
• accept both POSIX locales like en_US.UTF-8 and simple language tags like en-US, then normalize to ll_CC.UTF-8
• support --remove for both timezone and language overrides
• clarify in docs that changing proxy does not silently rewrite existing tz / lang; users can resync explicitly with auto

Validation

• shellcheck -s bash -S warning src/utils.sh src/cmd_*.sh src/dns_block.sh src/mtls.sh src/templates.sh src/main.sh build.sh
• bash -n src/utils.sh src/cmd_*.sh src/dns_block.sh src/mtls.sh src/templates.sh src/main.sh build.sh
• node --check src/relay.js && node --check src/fingerprint-hook.js
• bash build.sh and verified no drift against committed cac
• local smoke test: timezone Asia/Shanghai writes through correctly
• local smoke test: language en-US normalizes to en_US.UTF-8
• local smoke test: invalid timezone and language inputs are rejected