Finish backup spec: button state, invariant assertion, NAS error surfacing

- BackupView: action button shows "Checking status…" with spinner and blocks
  taps while coordinator.phase == .checking — prevents starting a backup
  before reconciliation has determined what actually needs uploading

- BackupView: NAS status row shows statusService.refreshError (actual SMB
  error reason: auth failure, timeout, host unreachable) instead of the
  generic "Unreachable" string — surfaces the real diagnosis to the user

- BackupStatusService: invariant assertion after every full reconcile — logs
  .error if alreadySafe + needBackup ≠ phoneTotal so filter mismatches and
  off-by-ones surface immediately in Console.app

- BackupStatusService.buildManifestIndex: full diagnostic logging — remotePath,
  manifest file path, download byte count, decoded entry count, bootstrap
  fallback reason — all visible in Console.app filtered by "BackupStatus"

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Features/Backup/BackupView.swift

@@ -646,9 +646,11 @@ struct BackupView: View {
                         .animation(.spring(response: 0.3, dampingFraction: 0.8), value: nasStatusLabel)
                 }
                 if lanMonitor.nasReachable == false {
-                    Text("Unreachable")
+                    Text(statusService.refreshError ?? "Unreachable")
                         .font(.system(size: 9, weight: .regular))
                         .foregroundStyle(AppTheme.destructive.opacity(0.7))
+                        .lineLimit(2)
+                        .multilineTextAlignment(.trailing)
                 } else if lanMonitor.nasReachable == true && !engine.job.status.isActive {
                     Text("Connected")
                         .font(.system(size: 9, weight: .regular))
@@ -691,43 +693,56 @@ struct BackupView: View {
 
     private var actionButton: some View {
         Group {
-            switch resolvedStatus {
-            case .idle, .completed, .failed, .cancelled:
-                Button(action: startBackup) {
-                    Text(resolvedStatus == .idle ? "Start backup" : "Back up again")
-                }
-                .buttonStyle(PrimaryButtonStyle(
-                    color: ctaFlashGreen ? AppTheme.positive : AppTheme.ink
-                ))
-                .accessibilityHint("Starts backing up your photos to the NAS")
-
-            case .running:
-                Button { engine.pause() } label: { Text("Pause") }
-                    .buttonStyle(GhostButtonStyle())
-                    .accessibilityLabel("Pause backup")
-
-            case .paused:
-                HStack(spacing: 10) {
-                    Button { engine.resume() } label: { Text("Resume") }
-                        .buttonStyle(PrimaryButtonStyle())
-                        .accessibilityLabel("Resume backup")
-                    Button { engine.cancel() } label: { Text("Cancel") }
-                        .buttonStyle(GhostButtonStyle())
-                        .frame(width: 90)
-                        .accessibilityLabel("Cancel backup")
-                }
-
-            case .preparing:
+            // Coordinator is running a status check — block the start button so
+            // a backup cannot be triggered before reconciliation completes.
+            if coordinator.phase == .checking {
                 HStack(spacing: 8) {
                     ProgressView().scaleEffect(0.8).tint(AppTheme.inkSecondary)
-                    Text("Preparing…")
+                    Text("Checking status…")
                         .font(AppTheme.body())
                         .foregroundStyle(AppTheme.inkSecondary)
                 }
                 .frame(height: 50)
+            } else {
+                switch resolvedStatus {
+                case .idle, .completed, .failed, .cancelled:
+                    Button(action: startBackup) {
+                        Text(resolvedStatus == .idle ? "Start backup" : "Back up again")
+                    }
+                    .buttonStyle(PrimaryButtonStyle(
+                        color: ctaFlashGreen ? AppTheme.positive : AppTheme.ink
+                    ))
+                    .accessibilityHint("Starts backing up your photos to the NAS")
+
+                case .running:
+                    Button { engine.pause() } label: { Text("Pause") }
+                        .buttonStyle(GhostButtonStyle())
+                        .accessibilityLabel("Pause backup")
+
+                case .paused:
+                    HStack(spacing: 10) {
+                        Button { engine.resume() } label: { Text("Resume") }
+                            .buttonStyle(PrimaryButtonStyle())
+                            .accessibilityLabel("Resume backup")
+                        Button { engine.cancel() } label: { Text("Cancel") }
+                            .buttonStyle(GhostButtonStyle())
+                            .frame(width: 90)
+                            .accessibilityLabel("Cancel backup")
+                    }
+
+                case .preparing:
+                    HStack(spacing: 8) {
+                        ProgressView().scaleEffect(0.8).tint(AppTheme.inkSecondary)
+                        Text("Preparing…")
+                            .font(AppTheme.body())
+                            .foregroundStyle(AppTheme.inkSecondary)
+                    }
+                    .frame(height: 50)
+                }
             }
         }
         .animation(.spring(response: 0.3, dampingFraction: 0.82), value: engine.job.status == .running)
+        .animation(.spring(response: 0.3, dampingFraction: 0.82), value: coordinator.phase == .checking)
     }
 
     private func startBackup() {