web: extract drand countdown formatting into a shared, testable lib

[karagozemin]

formatCountdown and the quicknet epoch math currently live inside the useDrandCountdown React hook, so they can't be reused or unit-tested outside React.

Move the pure parts into src/lib/countdown.ts and add a formatDuration helper to src/lib/format.ts, with unit tests for both.

[DeveloperEmmy]

Hello and good day. I'm experienced and would love to resolve your issues in no time. Thank you

[Adeyemi-cmd]

Can i please work on this

[grantfox-oss]

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