chore: versiona cartella android/ e pubspec.lock

Rende il repo clonabile e buildabile da chiunque senza passi manuali:
- android/ rimosso da .gitignore: contiene applicationId, label, icone
  e MainActivity con il package corretto (com.davide.biteplan)
- pubspec.lock aggiunto: necessario per --enforce-lockfile nella build
- File generati/macchina-specifici già esclusi da android/.gitignore
  (.gradle/, local.properties, GeneratedPluginRegistrant.java)

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

android/build.gradle.kts

@@ -0,0 +1,24 @@
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+val newBuildDir: Directory =
+    rootProject.layout.buildDirectory
+        .dir("../../build")
+        .get()
+rootProject.layout.buildDirectory.value(newBuildDir)
+
+subprojects {
+    val newSubprojectBuildDir: Directory = newBuildDir.dir(project.name)
+    project.layout.buildDirectory.value(newSubprojectBuildDir)
+}
+subprojects {
+    project.evaluationDependsOn(":app")
+}
+
+tasks.register<Delete>("clean") {
+    delete(rootProject.layout.buildDirectory)
+}