Add desktop platform support (Linux, macOS, Windows)

[cmays90]

This PR adds runtime guards for mobile-only APIs so Hacki can run on desktop platforms. The primary motivation is Linux (specifically Arch Linux, where it is packaged via the AUR), though macOS and Windows should benefit from the same changes.

What's guarded:

• receive_sharing_intent — share extension is Android/iOS only; calls are now skipped on desktop
• ChromeSafariBrowser / flutter_inappwebview — desktop platforms fall back to launchUrl with LaunchMode.externalApplication
• webview_flutter (inline bottom sheet) — not shown on Linux, which has no webview backend
• webview_flutter (offline reading screen) — offline HTML viewing is skipped on Linux
• wakelock_plus — not supported on Linux; calls are no-op guarded
• QR code scanner — camera scanning is hidden on desktop; the existing "From File" import path still works
• flutter_email_sender — the Email option in the bug report dialog is hidden on desktop; the GitHub option remains

I specifically did not address any packaging scaffolding for any of the Desktop platforms. The AUR packages handles this for Arch Linux.