Refactor Nostr ID Bridge & Keychain Helper (#796)

* Extract each type to a separate file

* Nostr ID Bridge: Convert static func/vars to instance

* `KeychainHelper` behind a protocol to easily mock

* Update tests with ID Bridge and MockKeychainHelper

---------

Co-authored-by: jack <212554440+jackjackbits@users.noreply.github.com>

bitchat/Services/LocationNotesManager.swift

@@ -14,6 +14,8 @@ struct LocationNotesDependencies {
     var sendEvent: SendEvent
     var deriveIdentity: (_ geohash: String) throws -> NostrIdentity
     var now: () -> Date
+    
+    private static let idBridge = NostrIdentityBridge()
 
     static let live = LocationNotesDependencies(
         relayLookup: { geohash, count in
@@ -35,7 +37,7 @@ struct LocationNotesDependencies {
             NostrRelayManager.shared.sendEvent(event, to: relays)
         },
         deriveIdentity: { geohash in
-            try NostrIdentityBridge.deriveIdentity(forGeohash: geohash)
+            try idBridge.deriveIdentity(forGeohash: geohash)
         },
         now: { Date() }
     )