Feat: Register `ed2k://` and `magnet:` URI schemes automatically

[ngosang]

Summary

aMule should register itself as the OS-level handler for both the ed2k:// and magnet: URI schemes automatically, the same way auto-start was made cross-platform in #744 — with a toggle in Preferences and an option in the installer — instead of requiring users to edit the registry, LaunchServices, or .desktop/xdg-mime by hand.

Current state

The code already fully supports handling these links once they reach aMule:

• CMagnetED2KConverter (src/MagnetURI.cpp) converts an eD2k-compatible magnet (xt=urn:ed2k:/urn:ed2khash: + xl=) into an ed2k:// link.
• CamuleAppCommon::CheckPassedLink() and InitCommon() (src/amuleAppCommon.cpp) accept both ed2k:// and magnet: links passed on the command line and queue them via the ED2KLinks file.
• The standalone ed2k tool (src/ED2KLinkParser.cpp) accepts magnet: and ed2k:// links.
• amulecmd's Add command accepts both as well.

What is missing is the OS-level scheme registration:

• Linux: org.amule.aMule.desktop declares only MimeType=x-scheme-handler/ed2k;. There is no x-scheme-handler/magnet, so magnet links from the browser never reach aMule. The ed2k association is also static (shipped in the .desktop); the user cannot toggle it.
• Windows: packaging/windows/installer.nsi registers no URL protocol at all — neither ed2k nor magnet. Users must import a .reg file manually.
• macOS: there is no Info.plist/CFBundleURLSchemes declaration; the OS only picks aMule up opportunistically through LaunchServices.

This is currently documented as a manual procedure on the website(User Manual → Utilities → ed2k → Browser Configuration). https://amule-org.github.io/docs/manual/utilities/ed2k

Requested change

Add automatic registration of the ed2k:// and magnet: schemes, implemented analogously to the auto-start feature from #744:

1. A cross-platform manager class (mirroring AutostartManager), with the OS being the source of truth:

   • Windows: HKCU\Software\Classes\<scheme> URL-protocol keys pointing at the aMule (or ed2k) binary — per-user, no elevation.
   • Linux: register x-scheme-handler/ed2k and x-scheme-handler/magnet (via .desktop MimeType= + the per-user
     mimeapps.list/xdg-mime).
   • macOS: declare both schemes in the app bundle's Info.plist (CFBundleURLTypes) and/or register via LaunchServices.
   • Enable() / Disable() / IsEnabled() per scheme, idempotent, plus a SelfHeal-style fixup so a moved binary keeps working (same pattern as AutostartManager::SelfHealOnStartup()).

2. A Preferences toggle (same place/style as the auto-start checkbox in PrefsUnifiedDlg) to enable/disable the associations, with a one-shot command-line option to match --configure-autostart (e.g. --configure-protocols on|off).

3. An installer option (Windows NSIS Components page, analogous to the existing auto-start checkbox) that registers the schemes on install and cleans them up on uninstall.

Notes

• magnet handling must keep its existing eD2k-compatibility requirement (magnet must carry xt=urn:ed2k:/urn:ed2khash: and xl=); BitTorrent-only magnets are out of scope.
• Once shipped, the manual browser-setup section in the docs should be reduced => https://amule-org.github.io/docs/manual/utilities/ed2k

Reference implementation to mirror: #744 (cross-platform auto-start).

[ngosang]

Update: The web browser should probably open “amule” or “amulegui” directly instead of the ‘ed2k’ executable. This has the advantage of opening the aMule window, allowing the user to see any errors that occur at that moment. The “ed2k” executable can remain unchanged.