fix(macOS): re-apply icon after app update (#9951)

## macOS: Re-apply custom icon after app build changes

### Summary

Fixes the regression where custom icons are not re-applied after app
updates, particularly affecting users on the tip channel.

### Problem

PR #9670 introduced caching to avoid redundantly setting the app icon on
every launch. However when Ghostty updates, the app bundle is replaced
and the custom icon is reset by macOS. Since `UserDefaults` persists
across updates, the cached icon name still matches the desired icon
name, causing the icon update to be incorrectly skipped.

This was reported by users in #9670 comments as well, that after
updating Ghostty the custom icon would disappear and require manual
re-application.

### Solution

- Track the app build number (`CFBundleVersion`) alongside the icon name
in `UserDefaults`
- Re-apply the icon if either the icon config has changed OR the build
number has changed
- Only update `UserDefaults` if `NSWorkspace.setIcon()` succeeds,
preventing false-positive caching on failure

I used `CFBundleVersion` (build number, e.g. `13509`) rather than
`CFBundleShortVersionString` (e.g. `1.2.3`) because tip builds increment
the build number with each release while the short version string
remains constant. I considered combining both but this seemed redundant.

### Related

- Fixes regression mentioned in comments on #9670
- Original issue: #9666
- Original discussion: #9665

macos/Sources/App/macOS/AppDelegate.swift

@@ -982,9 +982,15 @@ class AppDelegate: NSObject,
             appIconName = (colorStrings + [config.macosIconFrame.rawValue])
                 .joined(separator: "_")
         }
-        // Only change the icon if it has actually changed
-        // from the current one
-        guard UserDefaults.standard.string(forKey: "CustomGhosttyIcon") != appIconName else {
+
+        // Only change the icon if it has actually changed from the current one,
+        // or if the app build has changed (e.g. after an update that reset the icon)
+        let cachedIconName = UserDefaults.standard.string(forKey: "CustomGhosttyIcon")
+        let cachedIconBuild = UserDefaults.standard.string(forKey: "CustomGhosttyIconBuild")
+        let currentBuild = Bundle.main.infoDictionary?["CFBundleVersion"] as? String
+        let buildChanged = cachedIconBuild != currentBuild
+
+        guard cachedIconName != appIconName || buildChanged else {
 #if DEBUG
             if appIcon == nil {
                 await MainActor.run {
@@ -1001,14 +1007,16 @@ class AppDelegate: NSObject,
         let newIcon = appIcon
 
         let appPath = Bundle.main.bundlePath
-        NSWorkspace.shared.setIcon(newIcon, forFile: appPath, options: [])
+        guard NSWorkspace.shared.setIcon(newIcon, forFile: appPath, options: []) else { return }
         NSWorkspace.shared.noteFileSystemChanged(appPath)
 
         await MainActor.run {
             self.appIcon = newIcon
             NSApplication.shared.applicationIconImage = newIcon
         }
+
         UserDefaults.standard.set(appIconName, forKey: "CustomGhosttyIcon")
+        UserDefaults.standard.set(currentBuild, forKey: "CustomGhosttyIconBuild")
     }
 
     //MARK: - Restorable State