feat(shared): add opt-in diagnostics client with compile-time kill switch

Ship client-side telemetry, crash capture, and bug-report assembly behind a
user opt-in, with NoOp transport until Cloudflare is wired. Builds can set
vnidrop.diagnostics.included=false to omit the Share-diagnostics UI and
disable the telemetry/crash auto-upload stack while keeping bug reports.

shared/build.gradle.kts

@@ -37,6 +37,39 @@ plugins {
 	alias(libs.plugins.kotlinAtomicfu)
 }
 
+// Compile-time switch (gradle.properties or -Pvnidrop.diagnostics.included=false).
+// false: no Share-diagnostics toggle, no telemetry/crash auto-upload stack.
+val diagnosticsIncluded: Boolean =
+	(findProperty("vnidrop.diagnostics.included") as String?)?.toBooleanStrictOrNull() ?: true
+
+val diagnosticsBuildConfigDir = layout.buildDirectory.dir("generated/diagnostics/commonMain/kotlin")
+val generateDiagnosticsBuildConfig by tasks.registering {
+	group = "build"
+	description = "Generates DiagnosticsBuildConfig from vnidrop.diagnostics.included"
+	val outputDir = diagnosticsBuildConfigDir
+	val included = diagnosticsIncluded
+	inputs.property("vnidrop.diagnostics.included", included)
+	outputs.dir(outputDir)
+	doLast {
+		val packageDir = outputDir.get().asFile.resolve("com/vnidrop/app/diagnostics")
+		packageDir.mkdirs()
+		packageDir.resolve("DiagnosticsBuildConfig.kt").writeText(
+			"""
+			|package com.vnidrop.app.diagnostics
+			|
+			|/**
+			| * Generated by shared/build.gradle.kts.
+			| * Override with `-Pvnidrop.diagnostics.included=false` or gradle.properties.
+			| */
+			|object DiagnosticsBuildConfig {
+			|	const val INCLUDED: Boolean = $included
+			|}
+			|
+			""".trimMargin(),
+		)
+	}
+}
+
 kotlin {
 	if (GobleyHost.current.platform == GobleyHost.Platform.MacOS) {
 		listOf(
@@ -59,6 +92,9 @@ kotlin {
 	jvm()
 
 	sourceSets {
+		commonMain {
+			kotlin.srcDir(files(diagnosticsBuildConfigDir).builtBy(generateDiagnosticsBuildConfig))
+		}
 		androidMain.dependencies {
 			implementation(libs.androidx.activity.compose)
 			implementation(libs.androidx.core.ktx)