fix(geohash): publish currentGeohash writes to the threads that read it

[Chessing234]

GeohashRepository synchronizes every accessor that touches its shared maps — eighteen of them — but currentGeohash is a plain var with two unsynchronized accessors:

private var currentGeohash: String? = null

fun setCurrentGeohash(geo: String?) { currentGeohash = geo }
fun getCurrentGeohash(): String? = currentGeohash

It is written from the UI thread when the user picks a channel (GeohashViewModel.selectLocationChannel, MainActivity) and read from Nostr handler and timer threads. Several of those reads are inside the @Synchronized methods — refreshGeohashPeople, updateParticipant, displayNameForNostrPubkey — and holding the lock on the read side buys nothing when the write side never takes it: there is no happens-before edge, so a stale value can persist indefinitely.

Two places that shows up:

• startGeoParticipantsTimer loops while (repo.getCurrentGeohash() != null), so a missed write keeps refreshing participants for a channel the user has left;
• displayNameForNostrPubkey uses it to derive the identity that decides whether a pubkey is us, so a stale geohash can label someone else's message with your nickname, or drop yours.

Synchronizing the two accessors matches the rest of the class. NotificationManager holds the same state and marks it @Volatile for this reason, so the codebase already treats it as cross-thread state.

./gradlew :app:testDebugUnitTest :app:lintDebug passes locally (rc=0). No behaviour test: a missing happens-before edge cannot be observed reliably from a unit test, and asserting it would only encode a timing accident.