android: first-run onboarding and permissions screen as in the Final 2026 mockup

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -141,6 +141,10 @@
 			android:name=".ui.DiagnosticsActivity"
 			android:label="@string/diag_title">
 		</activity>
+		<activity
+			android:name=".ui.OnboardingActivity"
+			android:label="@string/app_name">
+		</activity>
         <activity
             android:name=".ui.RemediationInstructionsActivity"
             android:label="@string/remediation_instructions_title" >