build: desktopName + Linux snap channel config

electron-builder emits a pack-time warning on every Linux build that
desktopName is unset, which means the installed .desktop entry can't
be matched to a running window by most desktop environments (no
WM_CLASS / app_id link, so clicking the launcher icon while the app
is open does not raise / focus the window).

  - package.json: add top-level 'desktopName': 'MarkdownConverter'
    so Electron exposes the same value as its X11/Wayland window
    class and the .desktop file matches it.
  - electron-builder.config.js: mirror it under linux.* with
    syncDesktopName: true so the generated .desktop / .deb / snap
    entries all use the same identifier.
  - Add snap channel 'latest/edge' so the auto-updater feed from
    GitHub releases maps to a track snapd recognises. 'stable' would
    publish to a manually-managed channel and break the updater.

No source code changes; build only. Verified electron-builder accepts
the new keys without warnings on next pack.

Amit Haridas

package.json

@@ -3,6 +3,7 @@
   "version": "4.13.0",
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
+  "desktopName": "MarkdownConverter",
   "scripts": {
     "start": "electron .",
     "test": "cross-env NODE_OPTIONS=--experimental-vm-modules jest",