feat: surface check/revert/version in the menu and badge the icon

AppModel exposes appVersion, previousVersion, isCheckingForUpdates and
updateStatusMessage (an alias for the existing statusLine plumbing — one
status channel, not a new one), plus checkForUpdates() and
revertToPreviousVersion() wired to the hardened UpdateChecker.

Menu gains, in order: "Update to X" (unchanged, staged-only), "Check for
updates" (labelled "Checking…" and disabled mid-check), "Revert to <version>"
(only when a rollback copy exists), then the existing rows unchanged, then a
non-interactive footer "Redline <version>" with the status line under it —
same caption/secondary styles already used elsewhere in the file, no new
tokens.

Menu-bar icon gets a small badge while an update is staged: uses the SF
Symbol's own ".badge" variant when one exists for the current icon, otherwise
overlays a small dot on the plain symbol. Reads live model state, so the
badge disappears on its own once the offer clears.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Sources/Shotdeck/main.swift

@@ -21,8 +21,9 @@ struct ShotdeckApp: App {
                 .environment(appDelegate.model)
         } label: {
             let state = appDelegate.model.iconState
+            let hasUpdate = appDelegate.model.updateAvailable != nil
             HStack(spacing: 4) {
-                Image(systemName: state.symbolName)
+                menuBarIcon(for: state, hasUpdate: hasUpdate)
                 if let count = state.countText {
                     Text(count).font(.system(size: 11, weight: .semibold))
                 }
@@ -33,6 +34,29 @@ struct ShotdeckApp: App {
     }
 }
 
+/// The menu-bar symbol for `state`, badged while an update is staged. Uses the
+/// SF Symbol's own `.badge` variant when one exists; falls back to a small
+/// overlaid dot on the plain symbol otherwise. The badge disappears on its own
+/// once `updateAvailable` clears, since this reads live model state.
+@ViewBuilder
+private func menuBarIcon(for state: MenuIconState, hasUpdate: Bool) -> some View {
+    if hasUpdate {
+        let badgeName = "\(state.symbolName).badge"
+        if NSImage(systemSymbolName: badgeName, accessibilityDescription: nil) != nil {
+            Image(systemName: badgeName)
+        } else {
+            ZStack(alignment: .topTrailing) {
+                Image(systemName: state.symbolName)
+                Circle()
+                    .frame(width: 6, height: 6)
+                    .offset(x: 3, y: -3)
+            }
+        }
+    } else {
+        Image(systemName: state.symbolName)
+    }
+}
+
 @MainActor
 final class AppDelegate: NSObject, NSApplicationDelegate {
     let model: AppModel