🐛 Support Unicode site names in graphs

[AnandChowdhary]

Summary

• Adds shared slug resolution for graph generation: explicit site.slug → ASCII slugified name → sanitized Unicode site name → URL fallback.
• Preserves non-ASCII site names such as 鸭鸭梨 when ASCII slugification returns an empty value.
• Adds a regression test covering graph generation for a pure CJK site name.

Fixes upptime/upptime#933

Test plan

• PATH=/tmp/node-v14.21.3-linux-x64/bin:$PATH npm ci
• PATH=/tmp/node-v14.21.3-linux-x64/bin:$PATH npm test -- --runInBand src/index.spec.ts
• PATH=/tmp/node-v14.21.3-linux-x64/bin:$PATH npm run build
• PATH=/tmp/node-v14.21.3-linux-x64/bin:$PATH npm test
• PATH=/tmp/node-v14.21.3-linux-x64/bin:$PATH npm pack --dry-run

Notes

• This follows the Unicode slug fallback pattern already used by upptime/uptime-monitor.
• Local validation used Node 14 to match the legacy canvas/chartjs dependency stack.