fix: persist download queue to survive app restart (v1.6.1)

- Download queue now persisted to SharedPreferences
- Auto-restore pending downloads on app restart
- Interrupted downloads reset to queued and auto-resumed
- singleTask launch mode to prevent app restart on share intent
- onNewIntent handler for proper intent handling
- Reverted share_plus to 10.1.4 (12.0.1 has Kotlin build issues)

pubspec.lock

@@ -876,18 +876,18 @@ packages:
     dependency: "direct main"
     description:
       name: share_plus
-      sha256: "14c8860d4de93d3a7e53af51bff479598c4e999605290756bbbe45cf65b37840"
+      sha256: fce43200aa03ea87b91ce4c3ac79f0cecd52e2a7a56c7a4185023c271fbfa6da
       url: "https://pub.dev"
     source: hosted
-    version: "12.0.1"
+    version: "10.1.4"
   share_plus_platform_interface:
     dependency: transitive
     description:
       name: share_plus_platform_interface
-      sha256: "88023e53a13429bd65d8e85e11a9b484f49d4c190abbd96c7932b74d6927cc9a"
+      sha256: cc012a23fc2d479854e6c80150696c4a5f5bb62cb89af4de1c505cf78d0a5d0b
       url: "https://pub.dev"
     source: hosted
-    version: "6.1.0"
+    version: "5.0.2"
   shared_preferences:
     dependency: "direct main"
     description: