fix: correct link highlighting, update app store validation

Gaze/Services/AppStoreDetector.swift

@@ -9,6 +9,12 @@ import Foundation
 
 enum AppStoreDetector {
     /// Returns true if the app was downloaded from the Mac App Store
+    ///
+    /// Uses a heuristic approach that checks for the presence of a valid App Store receipt.
+    /// This is sufficient for distinguishing App Store builds from direct distribution.
+    ///
+    /// Note: This does not perform full cryptographic validation of the receipt signature, its
+    /// only used to determine if we should show the 'buy me a coffee' link.
     static var isAppStoreVersion: Bool {
         #if DEBUG
             return false
@@ -17,12 +23,30 @@ enum AppStoreDetector {
                 return false
             }
 
-            // Check if receipt exists and is in the expected App Store location
-            let fileManager = FileManager.default
-            let receiptExists = fileManager.fileExists(atPath: receiptURL.path)
-            let isInMASReceipt = receiptURL.path.contains("_MASReceipt")
+            guard FileManager.default.fileExists(atPath: receiptURL.path) else {
+                return false
+            }
 
-            return receiptExists && isInMASReceipt
+            guard let receiptData = try? Data(contentsOf: receiptURL),
+                receiptData.count > 2
+            else {
+                return false
+            }
+
+            // Verify receipt has PKCS#7 signature format (starts with ASN.1 SEQUENCE tag)
+            let bytes = [UInt8](receiptData.prefix(2))
+            return bytes[0] == 0x30 && bytes[1] == 0x82
+        #endif
+    }
+
+    /// Checks if the app is running in TestFlight
+    ///
+    /// TestFlight builds have a receipt named "sandboxReceipt" instead of "receipt"
+    static var isTestFlight: Bool {
+        #if DEBUG
+            return false
+        #else
+            return Bundle.main.appStoreReceiptURL?.lastPathComponent == "sandboxReceipt"
         #endif
     }
 }

Gaze/Views/Onboarding/PostureSetupView.swift

@@ -5,8 +5,8 @@
 //  Created by Mike Freno on 1/7/26.
 //
 
-import SwiftUI
 import AppKit
+import SwiftUI
 
 struct PostureSetupView: View {
     @Binding var enabled: Bool
@@ -33,9 +33,11 @@ struct PostureSetupView: View {
             VStack(spacing: 30) {
                 HStack(spacing: 12) {
                     Button(action: {
+                        // Using properly URL-encoded text fragment
+                        // Points to key findings about sitting posture and behavior relationship with LBP
                         if let url = URL(
                             string:
-                                "https://pubmed.ncbi.nlm.nih.gov/40111906/#:~:text=For studies exploring sitting posture, seven found a relationship with LBP. Regarding studies on sitting behavior, only one showed no relationship between LBP prevalence, while twelve indicated a relationship."
+                                "https://pubmed.ncbi.nlm.nih.gov/40111906/#:~:text=For%20studies%20exploring%20sitting%20posture%2C%20seven%20found%20a%20relationship%20with%20LBP.%20Regarding%20studies%20on%20sitting%20behavior%2C%20only%20one%20showed%20no%20relationship%20between%20LBP%20prevalence"
                         ) {
                             #if os(iOS)
                                 UIApplication.shared.open(url)
@@ -96,7 +98,7 @@ struct PostureSetupView: View {
                     .font(.caption)
                     .foregroundColor(.secondary)
                 }
-                
+
                 // Preview button
                 Button(action: {
                     showPreviewWindow()
@@ -120,34 +122,34 @@ struct PostureSetupView: View {
         .padding()
         .background(.clear)
     }
-    
+
     private func showPreviewWindow() {
         guard let screen = NSScreen.main else { return }
-        
+
         let window = NSWindow(
             contentRect: screen.frame,
             styleMask: [.borderless, .fullSizeContentView],
             backing: .buffered,
             defer: false
         )
-        
+
         window.level = .floating
         window.isOpaque = false
         window.backgroundColor = .clear
         window.collectionBehavior = [.canJoinAllSpaces, .fullScreenAuxiliary]
         window.acceptsMouseMovedEvents = true
-        
+
         let contentView = PostureReminderView(sizePercentage: 10.0) { [weak window] in
             window?.close()
         }
-        
+
         window.contentView = NSHostingView(rootView: contentView)
         window.makeFirstResponder(window.contentView)
-        
+
         let windowController = NSWindowController(window: window)
         windowController.showWindow(nil)
         window.makeKeyAndOrderFront(nil)
-        
+
         previewWindowController = windowController
     }
 }

GazeTests/Services/AppStoreDetectorTests.swift

@@ -9,9 +9,21 @@
 import Testing
 
 struct AppStoreDetectorTests {
-    
+
     @Test func isAppStoreVersionReturnsFalseInDebug() {
         // In test/debug builds, should always return false
         #expect(AppStoreDetector.isAppStoreVersion == false)
     }
+
+    @Test func isTestFlightReturnsFalseInDebug() {
+        // In test/debug builds, should always return false
+        #expect(AppStoreDetector.isTestFlight == false)
+    }
+
+    @Test func receiptValidationHandlesMissingReceipt() {
+        // When there's no receipt (development build), should return false
+        // This is implicitly tested by isAppStoreVersionReturnsFalseInDebug
+        // but we're documenting the expected behavior
+        #expect(AppStoreDetector.isAppStoreVersion == false)
+    }
 }