docs: explain city rank

[therunnas]

What does this PR do?

Adds a short, shared explanation of what City Rank means and reuses it where rank is currently shown without enough context.

Changes:

• add src/lib/city-rank.ts with shared rank copy
• show the explanation under the rank badge on /dev/[username], with title and aria-label on the badge
• clarify weekly and monthly digest labels/body text so email recipients understand rank is based on total GitHub contributions

Related issue

Closes #71

Screenshots

Local verification on /dev/dev_001:

The Vercel preview currently requires maintainer authorization because this PR comes from a fork.

Checklist

• npm run lint passes - full repo lint currently fails on pre-existing baseline errors outside this PR
• Tested locally - scoped ESLint passes on the changed files and the profile page renders the helper text
• No secrets or .env values committed

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