build(meezi_app): Android Maven mirrors for Iran (Aliyun)

Google's Android maven2 artifacts (AGP, androidx, Kotlin) 404 from Iran like
pub.dev does. Route Gradle resolution through the reachable Aliyun mirrors:

- android/settings.gradle.kts (pluginManagement) + build.gradle.kts (allprojects)
  now list maven.aliyun.com/repository/{gradle-plugin,google,central} before the
  originals (kept as fallback).
- BUILD_IRAN.md documents the full setup incl. the machine-local GRADLE_USER_HOME
  init.gradle needed for Flutter's included flutter_tools/gradle build.

Verified: dependency resolution now succeeds via the mirrors (AGP + kotlin-compiler
download from Aliyun). The APK build itself is currently blocked only by low disk
space on this machine, not configuration.

mobile/meezi_app/android/build.gradle.kts

@@ -1,5 +1,8 @@
 allprojects {
     repositories {
+        // Iran: prefer reachable Aliyun mirrors (Google Android maven2 is filtered here).
+        maven { url = uri("https://maven.aliyun.com/repository/google") }
+        maven { url = uri("https://maven.aliyun.com/repository/central") }
         google()
         mavenCentral()
     }