desktop: don't delete shared temp dirs from a transient second instance (#7168)

On an incoming call the desktop window blinked continuously (FileNotFoundException
from SoundPlayer.start writing into a missing ...\Temp\simplex).

Root cause: Files.desktop top-level val initializers run in any process that
touches the facade class, including a transient second instance (which reaches
acquireSingleInstance -> dataDir). Two of them deleted shared state used by the
running primary:
- tmpDir had .also { deleteOnExit() } - the second instance's normal exit deleted
  ...\Temp\simplex, so the primary then failed to write call sounds/recordings.
- preferencesTmpDir had .also { deleteRecursively() } - the second instance's
  <clinit> wiped configPath\tmp (same anti-pattern, firing even earlier).

Make both declarations pure and do the destructive cleanup in Main, past the
single-instance check, so only the owning instance performs it (preferencesTmpDir
kept early, before any settings write). The remaining val-initializer side effects
are idempotent creations (mkdirs) that destroy nothing and are left in place.

apps/multiplatform/common/src/desktopMain/kotlin/chat/simplex/common/platform/Files.desktop.kt

@@ -10,15 +10,18 @@ import java.io.*
 import java.net.URI
 
 actual val dataDir: File = File(desktopPlatform.dataPath)
-actual val tmpDir: File = File(System.getProperty("java.io.tmpdir") + File.separator + "simplex").also { it.deleteOnExit() }
+// No deleteOnExit() here: a transient second instance also inits this val, and its exit
+// would delete the shared folder while the primary runs. Registered in Main instead.
+actual val tmpDir: File = File(System.getProperty("java.io.tmpdir") + File.separator + "simplex")
 actual val filesDir: File = File(dataDir.absolutePath + File.separator + "simplex_v1_files")
 actual val appFilesDir: File = filesDir
 actual val wallpapersDir: File = File(dataDir.absolutePath + File.separator + "simplex_v1_assets" + File.separator + "wallpapers").also { it.mkdirs() }
 actual val coreTmpDir: File = File(dataDir.absolutePath + File.separator + "tmp")
 actual val dbAbsolutePrefixPath: String = dataDir.absolutePath + File.separator + "simplex_v1"
 actual val preferencesDir = File(desktopPlatform.configPath).also { it.parentFile.mkdirs() }
+// No deleteRecursively() here (see tmpDir): a second instance would wipe this shared
+// folder while the primary runs. Cleaned in Main instead.
 actual val preferencesTmpDir = File(desktopPlatform.configPath, "tmp")
-  .also { it.deleteRecursively() }
 
 actual val chatDatabaseFileName: String = "simplex_v1_chat.db"
 actual val agentDatabaseFileName: String = "simplex_v1_agent.db"