feat: add optional country flags to node names

[pengzhendong]

Summary

This PR adds optional country flag support for generated subscription configs and improves country grouping coverage/localization.

Changes

• Add show_flags=true support to prefix detected node names with country/region flag emoji.
• Add a UI toggle for showing flags in node names.
• Preserve existing node names by default when show_flags is not enabled.
• Avoid duplicating flags when node names already contain a flag emoji.
• Expand country/region detection from the previous limited list to 249 ISO country/region codes.
• Generate flag emoji from country/region codes instead of hardcoding every flag.
• Localize generated country group names based on the selected link language, for example 🇫🇮 芬兰 in Chinese and 🇫🇮 Finland in English.
• Add explicit lang parameter to generated links so downloaded configs keep the same language as the page instead of depending on client request headers.

Tests

• Added tests for country flag prefixing.
• Added coverage for Taiwan, Finland, and Hungary detection.
• Updated country grouping tests for localized names and full ISO coverage.
• Verified with:

npm test
npm run build

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