Migrate sharing to SharePlus.instance with ShareParams

[zensgit]

Steps:\n1) Upgrade share_plus to version supporting SharePlus.instance.share(ShareParams)\n2) Replace Share.share/shareXFiles usages with SharePlus.instance.share(ShareParams) in:\n - lib/widgets/qr_code_generator.dart\n - lib/services/share_service.dart (and other call-sites)\n3) Add minimal regression tests for text/files sharing paths\n\nNotes:\n- Keep behavior identical; step-1 might require constraints review\n- Split dependency bump and migration if needed