Update dependencies to latest + migrate to AGP 9.x

Kotlin 2.1.0->2.4.10, Compose Multiplatform 1.7.3->1.11.1, AGP 8.7.3->9.3.1, Gradle 8.10.2->9.7.1. The two libs that pinned us to Compose 1.7.x -- compose-webview-multiplatform (1.9.40->2.0.3) and multiplatform-markdown-renderer (0.33.0->0.44.0) -- now have Compose 1.8+ releases, so the whole stack moves. Also coroutines/serialization 1.11.0, okhttp 5.5.0, media3 1.11.0, activity-compose 1.13.0, firebase-messaging 25.1.2, compose-bom 2026.08.00, google-services 4.5.0, KCEF 2025.03.23 (JBR pin ->17.0.14); material3 1.9.0 / material-icons-extended 1.7.3 (each one's latest stable -- they lag the core).

AGP 9 migration: :shared swaps com.android.library->com.android.kotlin.multiplatform.library (android config moves into kotlin{android{}}, withHostTest{} keeps host tests); :androidApp drops kotlin(android) for AGP 9 built-in Kotlin. compileSdk 34->37 (minSdk stays 29 / Android 10).

Code fixes for API breaks: markdown link->textLink + highlights->highlightsBuilder; kotlinOptions{jvmTarget}->compilerOptions{jvmTarget}.

app/desktopApp/build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
     id("org.jetbrains.kotlin.plugin.compose")
 }
 
-val composeVersion = "1.7.3"
+val composeVersion = "1.11.1"
 val os = OperatingSystem.current()
 val arch = System.getProperty("os.arch") ?: "amd64"
 val desktopTarget = when {
@@ -19,7 +19,7 @@ dependencies {
     implementation(project(":shared"))
     implementation("org.jetbrains.compose.desktop:desktop-jvm-$desktopTarget:$composeVersion")
     // M6: window-state persistence (window.json).
-    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.7.3")
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.11.0")
 }
 
 compose.desktop {