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/src/jvmMain/kotlin/com/vnidrop/app/diagnostics/PendingCrashStore.jvm.kt

@@ -0,0 +1,36 @@
+package com.vnidrop.app.diagnostics
+
+import java.io.File
+import java.nio.charset.StandardCharsets
+
+actual fun createPendingCrashStore(appDataDir: String): PendingCrashStore =
+	JvmPendingCrashStore(appDataDir)
+
+private class JvmPendingCrashStore(
+	appDataDir: String,
+) : PendingCrashStore {
+	private val directory = File(appDataDir, "diagnostics/crashes")
+
+	@Synchronized
+	override fun write(report: CrashReport) {
+		directory.mkdirs()
+		File(directory, "${report.id}.crash").writeText(CrashReportCodec.encode(report), StandardCharsets.UTF_8)
+	}
+
+	@Synchronized
+	override fun list(): List<CrashReport> {
+		if (!directory.isDirectory) return emptyList()
+		return directory
+			.listFiles { file -> file.isFile && file.name.endsWith(".crash") }
+			.orEmpty()
+			.sortedByDescending { it.lastModified() }
+			.mapNotNull { file ->
+				runCatching { CrashReportCodec.decode(file.readText(StandardCharsets.UTF_8)) }.getOrNull()
+			}
+	}
+
+	@Synchronized
+	override fun delete(id: String) {
+		File(directory, "$id.crash").delete()
+	}
+}

shared/src/jvmMain/kotlin/com/vnidrop/app/diagnostics/PlatformCrashHook.jvm.kt

@@ -0,0 +1,9 @@
+package com.vnidrop.app.diagnostics
+
+actual fun installPlatformCrashHook(onCrash: (Throwable) -> Unit) {
+	val previous = Thread.getDefaultUncaughtExceptionHandler()
+	Thread.setDefaultUncaughtExceptionHandler { thread, throwable ->
+		runCatching { onCrash(throwable) }
+		previous?.uncaughtException(thread, throwable)
+	}
+}

shared/src/jvmMain/kotlin/com/vnidrop/app/logging/PlatformLogStore.jvm.kt

@@ -1,6 +1,7 @@
 package com.vnidrop.app.logging
 
 import java.io.File
+import java.io.RandomAccessFile
 import java.nio.charset.StandardCharsets
 
 actual fun createPlatformLogStore(appDataDir: String, policy: LogRotationPolicy): PlatformLogStore =
@@ -37,6 +38,32 @@ private class JvmPlatformLogStore(
 			.map { file -> LogFileInfo(file.name, file.absolutePath, file.length(), file.lastModified()) }
 	}
 
+	@Synchronized
+	override fun readLatest(maxBytes: Long): String {
+		if (maxBytes <= 0) return ""
+		directory.mkdirs()
+		val files = listOf(activeFile) +
+			(1..policy.maxFiles).map { File(directory, "app.$it.log") }
+		val chunks = ArrayList<ByteArray>()
+		var remaining = maxBytes
+		for (file in files) {
+			if (remaining <= 0 || !file.isFile) continue
+			val slice = readTail(file, remaining)
+			if (slice.isEmpty()) continue
+			chunks.add(0, slice)
+			remaining -= slice.size.toLong()
+		}
+		if (chunks.isEmpty()) return ""
+		val total = chunks.sumOf { it.size }
+		val out = ByteArray(total)
+		var offset = 0
+		for (chunk in chunks) {
+			chunk.copyInto(out, offset)
+			offset += chunk.size
+		}
+		return String(out, StandardCharsets.UTF_8)
+	}
+
 	private fun rotate() {
 		if (policy.maxFiles == 0) {
 			activeFile.delete()
@@ -54,3 +81,23 @@ private class JvmPlatformLogStore(
 		}
 	}
 }
+
+private fun readTail(file: File, maxBytes: Long): ByteArray {
+	if (!file.isFile || file.length() == 0L || maxBytes <= 0) return ByteArray(0)
+	val length = file.length()
+	val start = (length - maxBytes).coerceAtLeast(0L)
+	val size = (length - start).toInt()
+	RandomAccessFile(file, "r").use { raf ->
+		raf.seek(start)
+		val bytes = ByteArray(size)
+		raf.readFully(bytes)
+		if (start == 0L) return bytes
+		// Align to the next full line when we start mid-file.
+		val newline = bytes.indexOf('\n'.code.toByte())
+		return if (newline in 0 until bytes.lastIndex) {
+			bytes.copyOfRange(newline + 1, bytes.size)
+		} else {
+			bytes
+		}
+	}
+}