Add Copy JSON option to session copy icon

[Reasel]

Summary

The copy icon on each session history row now opens a menu with two options:

• Copy stats: the existing human-readable text, unchanged
• Copy JSON: the session serialized with Gson, for pasting into the afk-stats community site

Adds SessionJsonTest, a contract test that locks the JSON field names the site's API validates (startTime, endTime, clickCount, consistencyScore, avgInterval, avgDistancePercent, name).

Exported JSON rounds avgInterval and avgDistancePercent to two decimals (roundForExport); stored history keeps full precision.

Testing

• ./gradlew build green (SessionJsonTest x2 plus all existing unit tests)
• Manually verified in the dev client: menu opens, both copies land on the clipboard, JSON matches the contract