build: add kotlinx.serialization and Glance for M10

A pure-Kotlin JSON codec (so the backup round-trip test runs on the
JVM) and Jetpack Glance (the three M10 widgets). Both resolve from
google()/mavenCentral() only, no verification-metadata file exists to
update, and the repro guard checks vcsInfo/dependenciesInfo/foojay —
none of which this touches.

app/build.gradle.kts

@@ -4,6 +4,7 @@ import java.io.FileInputStream
 plugins {
     alias(libs.plugins.android.application)
     alias(libs.plugins.kotlin.compose)
+    alias(libs.plugins.kotlin.serialization)
     alias(libs.plugins.ksp)
     alias(libs.plugins.hilt)
 }
@@ -184,6 +185,12 @@ dependencies {
 
     implementation(libs.kotlinx.datetime)
     implementation(libs.kotlinx.coroutines.core)
+    implementation(libs.kotlinx.serialization.json)
+
+    // M10 home-screen widgets (docs/PLAN.md §9).
+    implementation(libs.androidx.glance.appwidget)
+    implementation(libs.androidx.glance.material3)
+
     implementation("de.jeanlucmakiola.floret:core-time")
     implementation("de.jeanlucmakiola.floret:core-locale")
     implementation("de.jeanlucmakiola.floret:core-crash")