chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/android/settings.gradle.kts

@@ -1,26 +0,0 @@
-pluginManagement {
-    val flutterSdkPath =
-        run {
-            val properties = java.util.Properties()
-            file("local.properties").inputStream().use { properties.load(it) }
-            val flutterSdkPath = properties.getProperty("flutter.sdk")
-            require(flutterSdkPath != null) { "flutter.sdk not set in local.properties" }
-            flutterSdkPath
-        }
-
-    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
-
-    repositories {
-        google()
-        mavenCentral()
-        gradlePluginPortal()
-    }
-}
-
-plugins {
-    id("dev.flutter.flutter-plugin-loader") version "1.0.0"
-    id("com.android.application") version "9.1.0" apply false
-    id("org.jetbrains.kotlin.android") version "2.4.0" apply false
-}
-
-include(":app")