fix: deliver runBrew completion on the main thread

Completions were invoked on a background queue. Most call sites wrapped
their UI work in DispatchQueue.main.async, but upgradeSingle's completion
calls refreshAll(), which sets statusMenuItem.title directly — an AppKit
mutation off the main thread. It also raced on cachedOutdated /
lastOutdatedFetch (written from background, read from main).

runBrew now always dispatches its completion to the main queue, so every
caller can safely touch UI, and the now-redundant inner main.async hops
are removed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

BrewBar/BrewBarApp.swift

@@ -194,7 +194,7 @@ class AppDelegate: NSObject, NSApplicationDelegate {
                 try process.run()
             } catch {
                 self.log("ERROR: \(error)\n")
-                completion("")
+                DispatchQueue.main.async { completion("") }
                 return
             }
 
@@ -204,7 +204,8 @@ class AppDelegate: NSObject, NSApplicationDelegate {
             let output = String(data: data, encoding: .utf8) ?? ""
 
             self.log(output)
-            completion(output)
+            // completion always on main: callers update AppKit UI, which is main-thread only
+            DispatchQueue.main.async { completion(output) }
         }
     }
 
@@ -249,7 +250,6 @@ class AppDelegate: NSObject, NSApplicationDelegate {
             self.cachedOutdated = lines
             self.lastOutdatedFetch = Date()
 
-            DispatchQueue.main.async {
             self.updateOutdatedMenu(with: lines)
             self.updateStatus(count: lines.count)
             if lines.isEmpty {
@@ -259,7 +259,6 @@ class AppDelegate: NSObject, NSApplicationDelegate {
             }
         }
     }
-    }
 
     func updateOutdatedMenu(with lines: [String]) {
         outdatedSubmenu.removeAllItems()
@@ -305,12 +304,9 @@ class AppDelegate: NSObject, NSApplicationDelegate {
     func refreshBrewVersion() {
         runBrew("--version") { output in
             let firstLine = BrewParser.parseVersion(output)
-
-            DispatchQueue.main.async {
             self.versionItem.title = "🏷 \(firstLine)"
         }
     }
-    }
 
     // MARK: - Logs