Update repository URLs to nextgen-networks while preserving deep links

Replaced bare references to the-djmaze/snappymail with nextgen-networks/snappymail/
and updated .git URLs to the new organization.

Per user request:
- README files are excluded.
- URLs with a trailing slash (e.g. issues, pull requests, releases) are
  left pointing to the original repository to preserve historical context.
- Bare web URLs now include the requested trailing slash.
- Existing nextgen-networks bare URLs were updated to include the trailing slash.

Co-authored-by: nextgen-networks <21206978+nextgen-networks@users.noreply.github.com>

package.json

@@ -25,7 +25,7 @@
     }
   ],
   "bugs": {
-    "url": "https://github.com/nextgen-networks/snappymail/issues"
+    "url": "https://github.com/the-djmaze/snappymail/issues"
   },
   "keywords": [
     "webmail",