- Refactor: Enable concurrent model downloads

This change refactors the model download service to support downloading multiple models simultaneously.

Key changes:
- Replaced the single `downloadState` with a `downloadStates` map, allowing individual tracking of each download's progress, keyed by model ID.
- Each download now runs in its own coroutine job and displays a separate notification, preventing conflicts.
- Cancellation is now specific to a model ID, allowing users to cancel individual downloads without affecting others.
- Implemented robust cleanup logic to delete temporary files and remove download state upon completion, cancellation, or error.
- Added a new `EmbeddingModelDownloadWorker` to handle the download of the embedding model in the background, separate from the main model download service.

Author: Saumesh425

app/build.gradle.kts

@@ -22,7 +22,7 @@ android {
         applicationId = "com.dark.tool_neuron"
         minSdk = 31
         targetSdk = 36
-        versionCode = 17
+        versionCode = 18
         versionName = "1.1.2-Fix"
         ndk {
             abiFilters += listOf("arm64-v8a", "x86_64")