Add media key interception, preferences UI, dynamic menu bar icon

- Play key (F8) launches preferred player when no player is running
- Blocks Apple Music auto-launch (hide + forceTerminate, like noTunes)
- Preferences window: player picker with app icons, "Choose Other App..."
- Known players: Spotify, Apple Music, Yandex Music, VLC, IINA, Vox
- Start at login via LaunchAgent
- Menu bar icon dynamically reflects current volume level and mute state

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

Sources/HUDini/LoginItemManager.swift

@@ -0,0 +1,41 @@
+import Foundation
+
+struct LoginItemManager {
+    private static let plistPath: String = {
+        let home = FileManager.default.homeDirectoryForCurrentUser.path
+        return "\(home)/Library/LaunchAgents/com.grosfrumos.hudini.plist"
+    }()
+
+    static var isEnabled: Bool {
+        FileManager.default.fileExists(atPath: plistPath)
+    }
+
+    static func setEnabled(_ enabled: Bool, appPath: String? = nil) {
+        if enabled {
+            let path = appPath ?? Bundle.main.bundlePath
+            let plist = """
+            <?xml version="1.0" encoding="UTF-8"?>
+            <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+            <plist version="1.0">
+            <dict>
+                <key>Label</key>
+                <string>com.grosfrumos.hudini</string>
+                <key>ProgramArguments</key>
+                <array>
+                    <string>open</string>
+                    <string>\(path)</string>
+                </array>
+                <key>RunAtLoad</key>
+                <true/>
+            </dict>
+            </plist>
+            """
+            // Ensure LaunchAgents dir exists
+            let dir = (plistPath as NSString).deletingLastPathComponent
+            try? FileManager.default.createDirectory(atPath: dir, withIntermediateDirectories: true)
+            try? plist.write(toFile: plistPath, atomically: true, encoding: .utf8)
+        } else {
+            try? FileManager.default.removeItem(atPath: plistPath)
+        }
+    }
+}