camera: requestAudioPermission — in-app prompt first, settings on permanent denial

Banner-tap entry point that shows the system mic prompt when the OS
will still surface one, and deep-links to Settings only on permanent
denial. Fixes the fresh-install trap where the mic entry isn't in the
Privacy pane until requestAccess has fired at least once.

Android tracks the first-asked state in SharedPreferences because
shouldShowRequestPermissionRationale returns false in two
observationally identical states (never asked vs permanently denied).
The existing initialize() request path writes the flag too, so a
banner tap after a record-then-deny correctly routes to Settings.

Refactored Android pendingPermission into PendingPermission(primary,
kind, cb) so audio-only requests check RECORD_AUDIO results instead
of always checking CAMERA.

darwin/Camera/CameraPlugin.swift

@@ -272,6 +272,27 @@ public class CameraPlugin: NSObject, NativePlugin, FlutterStreamHandler {
             let granted = AVCaptureDevice.authorizationStatus(for: .audio) == .authorized
             result(granted)
 
+        case "requestAudioPermission":
+            // On `.notDetermined` the system can still show the in-app
+            // prompt; on `.denied`/`.restricted` it can't — and on a
+            // fresh install the mic entry isn't in the Privacy pane
+            // until requestAccess has fired at least once, so the
+            // settings fallback only makes sense after first prompt.
+            let status = AVCaptureDevice.authorizationStatus(for: .audio)
+            switch status {
+            case .authorized:
+                result(true)
+            case .notDetermined:
+                AVCaptureDevice.requestAccess(for: .audio) { granted in
+                    DispatchQueue.main.async { result(granted) }
+                }
+            default:
+                DispatchQueue.main.async {
+                    CameraSettings.openAppSettings()
+                    result(false)
+                }
+            }
+
         case "openSettings":
             DispatchQueue.main.async {
                 // Per-platform helper: iOS opens app-specific Settings