Add mark-seen action, critical-alert badge, and a dedicated window0.5.0

Closes #8-#10 (milestone 0.5.0).

- Mark-seen: each row gets a checkmark button (separate from the
  click-to-open button) that persists the event into seenEventIDs and
  removes it from the active stream immediately, no API write.
- Menu bar icon switches to a filled exclamation-shield with a red badge
  showing the count of unseen critical alerts when any exist.
- Added a dedicated Window (singleton scene, not WindowGroup — avoids
  spawning duplicates) opened via a header button, sharing the exact
  same store/authStore instances as the popover. The button hides itself
  when already viewing the standalone window.

1 files changed, 32 insertions, 1 deletions

diff --git a/octosentry/octosentryApp.swift b/octosentry/octosentryApp.swift
index cf7522d..e80c4a0 100644
--- a/octosentry/octosentryApp.swift
+++ b/octosentry/octosentryApp.swift
@@ -7,15 +7,46 @@
 
 import SwiftUI
 
+enum SecurityEventWindow {
+    static let id = "security-events-window"
+}
+
 @main
 struct octosentryApp: App {
     @State private var store = SecurityEventStore()
     @State private var authStore = AuthStore()
 
     var body: some Scene {
-        MenuBarExtra("OctoSentry", systemImage: "shield.lefthalf.filled") {
+        MenuBarExtra {
             SecurityEventListView(store: store, authStore: authStore)
+                .frame(width: 380, height: 420)
+        } label: {
+            MenuBarIconView(criticalCount: store.unseenCriticalCount)
         }
         .menuBarExtraStyle(.window)
+
+        Window("Security Events", id: SecurityEventWindow.id) {
+            SecurityEventListView(store: store, authStore: authStore, isStandaloneWindow: true)
+                .frame(minWidth: 420, minHeight: 480)
+        }
+    }
+}
+
+private struct MenuBarIconView: View {
+    let criticalCount: Int
+
+    var body: some View {
+        ZStack(alignment: .topTrailing) {
+            Image(systemName: criticalCount > 0 ? "exclamationmark.shield.fill" : "shield.lefthalf.filled")
+
+            if criticalCount > 0 {
+                Text(criticalCount > 9 ? "9+" : "\(criticalCount)")
+                    .font(.system(size: 8, weight: .bold))
+                    .foregroundStyle(.white)
+                    .padding(2)
+                    .background(Circle().fill(.red))
+                    .offset(x: 8, y: -6)
+            }
+        }
     }
 }