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>

dev/View/Popup/Compose.js

@@ -968,7 +968,7 @@ export class ComposePopupView extends AbstractViewPopup {
 				// no default
 			}
 
-			// https://github.com/nextgen-networks/snappymail/issues/491
+			// https://github.com/the-djmaze/snappymail/issues/491
 			tpl.innerHTML = oLastMessage.bodyAsHTML();
 			tpl.content.querySelectorAll('img').forEach(img => {
 				img.src || img.dataset.xSrc || img.replaceWith(img.alt || img.title)

dev/View/Popup/Domain.js

@@ -269,7 +269,7 @@ export class DomainPopupView extends AbstractViewPopup {
 
 	testConnectionCommand() {
 		this.clearTesting();
-		// https://github.com/nextgen-networks/snappymail/issues/477
+		// https://github.com/the-djmaze/snappymail/issues/477
 		AskPopupView.credentials('IMAP', 'GLOBAL/TEST').then(credentials => {
 			if (credentials) {
 				this.testing(true);