Break circular dependency between CommandProcessor and ChatViewModel

Introduce CommandContextProvider protocol to define what CommandProcessor
needs from its context. This follows the Dependency Inversion Principle:

- Create CommandContextProvider protocol with 15 methods/properties
- Create CommandGeoParticipant struct for geo participant data
- Add getVisibleGeoParticipants() to ChatViewModel
- ChatViewModel now conforms to CommandContextProvider
- CommandProcessor depends on protocol, not concrete ChatViewModel

Benefits:
- Breaks the tight coupling between CommandProcessor and ChatViewModel
- Makes CommandProcessor more testable (can use mock context)
- Clear contract for what CommandProcessor needs
- Backwards-compatible via chatViewModel property alias

Also fixes a concurrency bug in BitchatApp where notification delegate
was accessing main-actor-isolated property from arbitrary thread.

bitchat/ViewModels/ChatViewModel.swift

@@ -92,7 +92,7 @@ import UniformTypeIdentifiers
 /// Manages the application state and business logic for BitChat.
 /// Acts as the primary coordinator between UI components and backend services,
 /// implementing the BitchatDelegate protocol to handle network events.
-final class ChatViewModel: ObservableObject, BitchatDelegate {
+final class ChatViewModel: ObservableObject, BitchatDelegate, CommandContextProvider {
     // Precompiled regexes and detectors reused across formatting
     private enum Regexes {
         static let hashtag: NSRegularExpression = {
@@ -1906,6 +1906,11 @@ final class ChatViewModel: ObservableObject, BitchatDelegate {
             .sorted { $0.lastSeen > $1.lastSeen }
         return people
     }
+
+    /// CommandContextProvider conformance - returns visible geo participants
+    func getVisibleGeoParticipants() -> [CommandGeoParticipant] {
+        visibleGeohashPeople().map { CommandGeoParticipant(id: $0.id, displayName: $0.displayName) }
+    }
     /// Returns the current participant count for a specific geohash, using the 5-minute activity window.
     @MainActor
     func geohashParticipantCount(for geohash: String) -> Int {