Show update-available banner automatically on launch (#1651)

* Show update-available banner automatically on launch

Probe for updates immediately on launch via Sparkle's
checkForUpdateInformation() so the sidebar surfaces a passive update
indicator without waiting for the 24h scheduler. When Sparkle detects
an available update in the background, the pill now shows
"Update Available: X.Y.Z" with accent styling while the updater is
idle. Clicking it triggers the full interactive update flow.

Also fixes thread safety in delegate callbacks by dispatching
@Published mutations to the main queue.

Closes #1643

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Add periodic background update probe every 15 minutes

The launch-only probe wouldn't catch updates published while the app
is already running. Add a repeating 15-minute timer that calls
checkForUpdateInformation() so the sidebar banner appears within a
reasonable window after a new version is published.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Change background update probe interval to 30 minutes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Change update check interval to 1 hour and migrate existing users

Reduce Sparkle's scheduled check interval from 24h to 1h so update
banners appear sooner. Migrate users stuck on the old 24h default by
bumping the migration key to v2. Align background probe interval with
the Sparkle check interval instead of hardcoding 30 minutes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Sources/Update/UpdateViewModel.swift

@@ -15,6 +15,14 @@ class UpdateViewModel: ObservableObject {
         overrideState ?? state
     }
 
+    var showsDetectedBackgroundUpdate: Bool {
+        effectiveState.isIdle && detectedUpdateVersion != nil
+    }
+
+    var showsPill: Bool {
+        !effectiveState.isIdle || showsDetectedBackgroundUpdate
+    }
+
     func recordDetectedUpdate(_ item: SUAppcastItem) {
         detectedUpdateVersion = Self.normalizedDetectedUpdateVersion(from: item.displayVersionString)
     }
@@ -27,6 +35,9 @@ class UpdateViewModel: ObservableObject {
         #if DEBUG
         if let debugOverrideText { return debugOverrideText }
         #endif
+        if let detectedText = detectedUpdateText {
+            return detectedText
+        }
         switch effectiveState {
         case .idle:
             return ""
@@ -60,6 +71,9 @@ class UpdateViewModel: ObservableObject {
     }
 
     var maxWidthText: String {
+        if let detectedText = detectedUpdateText {
+            return detectedText
+        }
         switch effectiveState {
         case .downloading:
             return "Downloading: 100%"
@@ -71,6 +85,9 @@ class UpdateViewModel: ObservableObject {
     }
 
     var iconName: String? {
+        if showsDetectedBackgroundUpdate {
+            return "shippingbox.fill"
+        }
         switch effectiveState {
         case .idle:
             return nil
@@ -135,6 +152,9 @@ class UpdateViewModel: ObservableObject {
     }
 
     var iconColor: Color {
+        if showsDetectedBackgroundUpdate {
+            return cmuxAccentColor()
+        }
         switch effectiveState {
         case .idle:
             return .secondary
@@ -154,6 +174,9 @@ class UpdateViewModel: ObservableObject {
     }
 
     var backgroundColor: Color {
+        if showsDetectedBackgroundUpdate {
+            return cmuxAccentColor()
+        }
         switch effectiveState {
         case .permissionRequest:
             return Color(nsColor: NSColor.systemBlue.blended(withFraction: 0.3, of: .black) ?? .systemBlue)
@@ -169,6 +192,9 @@ class UpdateViewModel: ObservableObject {
     }
 
     var foregroundColor: Color {
+        if showsDetectedBackgroundUpdate {
+            return .white
+        }
         switch effectiveState {
         case .permissionRequest:
             return .white
@@ -348,6 +374,11 @@ class UpdateViewModel: ObservableObject {
         let trimmed = version.trimmingCharacters(in: .whitespacesAndNewlines)
         return trimmed.isEmpty ? nil : trimmed
     }
+
+    private var detectedUpdateText: String? {
+        guard showsDetectedBackgroundUpdate, let version = detectedUpdateVersion else { return nil }
+        return String(localized: "update.available.withVersion", defaultValue: "Update Available: \(version)")
+    }
 }
 
 enum UpdateState: Equatable {