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

GeohashRepository synchronizes every accessor that touches its shared
maps - eighteen of them - but currentGeohash is a plain var with two
unsynchronized accessors, and it is written from the UI thread when the
user picks a channel while Nostr handler and timer threads read it.

Several of those reads happen 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.
Two ways that shows up: startGeoParticipantsTimer loops
`while (repo.getCurrentGeohash() != null)` and keeps refreshing a channel
the user has left, and displayNameForNostrPubkey derives the identity for
the wrong geohash when deciding whether a pubkey is us.

Synchronize the two accessors, matching the rest of the class.
NotificationManager keeps the same state @Volatile for this reason.

app/src/main/java/com/bitchat/android/nostr/GeohashRepository.kt

@@ -64,10 +64,17 @@ class GeohashRepository(
     // peerID alias -> nostr pubkey mapping for geohash DMs and temp aliases
     private val nostrKeyMapping: MutableMap<String, String> = mutableMapOf()
 
-    // Current geohash in view
+    // Current geohash in view.  Written from the UI thread when the user picks
+    // a channel and read from Nostr handler and timer threads - including from
+    // the @Synchronized methods below, which only see a published write if the
+    // write itself is synchronized.  NotificationManager marks its copy of this
+    // state @Volatile for the same reason.
     private var currentGeohash: String? = null
 
+    @Synchronized
     fun setCurrentGeohash(geo: String?) { currentGeohash = geo }
+
+    @Synchronized
     fun getCurrentGeohash(): String? = currentGeohash
 
     @Synchronized