Core-clean variant: editor id eurooffice, no text-app patch

- HtmlDirectEditor getId() now returns 'eurooffice' (hard-coded in the
  iOS/Android editor registry) -> 'Öffnen mit Office' long-press renders HTML
- install.sh restores the original TextDirectEditor if a previous
  install patched it; no core app patch anymore
- README rewritten: behavior, variant rationale, files

patches/htmlviewer/HtmlDirectEditor.php

@@ -34,7 +34,10 @@ class HtmlDirectEditor implements IEditor {
 
 	#[\Override]
 	public function getId(): string {
-		return Application::APP_ID;
+		// "eurooffice" is one of the hard-coded editor IDs the Nextcloud
+		// iOS/Android apps resolve in their editor registry. The menu
+		// "Öffnen mit Office" (long press) then opens this editor.
+		return 'eurooffice';
 	}
 
 	#[\Override]