Extract GeohashParticipantTracker from ChatViewModel

Refactor participant tracking logic into a dedicated, testable class:

- Create GeohashParticipantTracker with GeohashParticipantContext protocol
- Move GeoPerson struct to new file
- Extract participant recording, refresh, and count logic
- Add 18 comprehensive tests for the tracker
- Update ChatViewModel to use tracker via dependency injection
- Subscribe to tracker's objectWillChange for SwiftUI observation

This reduces coupling in ChatViewModel and makes participant tracking
testable in isolation.

bitchat/Services/GeohashParticipantTracker.swift

@@ -0,0 +1,159 @@
+//
+// GeohashParticipantTracker.swift
+// bitchat
+//
+// Tracks participants in geohash-based location channels.
+// This is free and unencumbered software released into the public domain.
+//
+
+import Foundation
+
+/// Represents a participant in a geohash channel
+public struct GeoPerson: Identifiable, Equatable, Sendable {
+    public let id: String        // pubkey hex (lowercased)
+    public let displayName: String
+    public let lastSeen: Date
+
+    public init(id: String, displayName: String, lastSeen: Date) {
+        self.id = id
+        self.displayName = displayName
+        self.lastSeen = lastSeen
+    }
+}
+
+/// Protocol for resolving display names and checking block status
+@MainActor
+public protocol GeohashParticipantContext: AnyObject {
+    /// Returns display name for a Nostr pubkey (e.g., "alice#a1b2" or "anon#c3d4")
+    func displayNameForPubkey(_ pubkeyHex: String) -> String
+    /// Returns true if the pubkey is blocked
+    func isBlocked(_ pubkeyHexLowercased: String) -> Bool
+}
+
+/// Tracks participants across multiple geohash channels
+@MainActor
+public final class GeohashParticipantTracker: ObservableObject {
+
+    /// Activity cutoff duration (defaults to 5 minutes)
+    public let activityCutoff: TimeInterval
+
+    /// Per-geohash participant map: [geohash: [pubkeyHex: lastSeen]]
+    private var participants: [String: [String: Date]] = [:]
+
+    /// Currently visible people for the active geohash
+    @Published public private(set) var visiblePeople: [GeoPerson] = []
+
+    /// The currently active geohash (if any)
+    private var activeGeohash: String?
+
+    /// Context for display name resolution and block checking
+    private weak var context: GeohashParticipantContext?
+
+    /// Timer for periodic refresh
+    private var refreshTimer: Timer?
+
+    public init(activityCutoff: TimeInterval = -300) { // default 5 minutes
+        self.activityCutoff = activityCutoff
+    }
+
+    /// Configure with a context provider
+    public func configure(context: GeohashParticipantContext) {
+        self.context = context
+    }
+
+    /// Set the currently active geohash
+    public func setActiveGeohash(_ geohash: String?) {
+        activeGeohash = geohash
+        if geohash == nil {
+            visiblePeople = []
+        } else {
+            refresh()
+        }
+    }
+
+    /// Record activity from a participant in the current active geohash
+    public func recordParticipant(pubkeyHex: String) {
+        guard let gh = activeGeohash else { return }
+        recordParticipant(pubkeyHex: pubkeyHex, geohash: gh)
+    }
+
+    /// Record activity from a participant in a specific geohash
+    public func recordParticipant(pubkeyHex: String, geohash: String) {
+        let key = pubkeyHex.lowercased()
+        var map = participants[geohash] ?? [:]
+        map[key] = Date()
+        participants[geohash] = map
+
+        // Only refresh visible list if this geohash is currently active
+        if activeGeohash == geohash {
+            refresh()
+        }
+    }
+
+    /// Remove a participant from all geohashes (used when blocking)
+    public func removeParticipant(pubkeyHex: String) {
+        let key = pubkeyHex.lowercased()
+        for (gh, var map) in participants {
+            map.removeValue(forKey: key)
+            participants[gh] = map
+        }
+        refresh()
+    }
+
+    /// Get participant count for a specific geohash
+    public func participantCount(for geohash: String) -> Int {
+        let cutoff = Date().addingTimeInterval(activityCutoff)
+        let map = participants[geohash] ?? [:]
+        return map.values.filter { $0 >= cutoff }.count
+    }
+
+    /// Get the visible people list for the active geohash (read-only query)
+    public func getVisiblePeople() -> [GeoPerson] {
+        guard let gh = activeGeohash, let context = context else { return [] }
+        let cutoff = Date().addingTimeInterval(activityCutoff)
+        let map = (participants[gh] ?? [:])
+            .filter { $0.value >= cutoff }
+            .filter { !context.isBlocked($0.key) }
+
+        return map
+            .map { (pub, seen) in
+                GeoPerson(id: pub, displayName: context.displayNameForPubkey(pub), lastSeen: seen)
+            }
+            .sorted { $0.lastSeen > $1.lastSeen }
+    }
+
+    /// Refresh the visible people list
+    public func refresh() {
+        visiblePeople = getVisiblePeople()
+    }
+
+    /// Start the periodic refresh timer
+    public func startRefreshTimer(interval: TimeInterval = 30.0) {
+        stopRefreshTimer()
+        refreshTimer = Timer.scheduledTimer(withTimeInterval: interval, repeats: true) { [weak self] _ in
+            Task { @MainActor in
+                self?.refresh()
+            }
+        }
+    }
+
+    /// Stop the periodic refresh timer
+    public func stopRefreshTimer() {
+        refreshTimer?.invalidate()
+        refreshTimer = nil
+    }
+
+    /// Clear all participant data
+    public func clear() {
+        participants.removeAll()
+        visiblePeople = []
+    }
+
+    /// Clear participant data for a specific geohash
+    public func clear(geohash: String) {
+        participants.removeValue(forKey: geohash)
+        if activeGeohash == geohash {
+            visiblePeople = []
+        }
+    }
+}