Fix code-review findings: ajax error handling, translated comment, docs

- js/index.js: the occ-command ajax call used the wrong jQuery .fail()
  signature ((response, code) instead of (jqXHR, textStatus,
  errorThrown)), so any failure printed the literal string
  "[object Object]" instead of a useful message.
- js/index.js: the initial bootstrap call that builds the terminal had
  no .fail() handler at all - a failure there (expired session/CSRF
  token, network error, rejected admin check) left #app-content
  permanently blank with zero diagnostic information. This is exactly
  what made today's NC34 debugging session harder than it needed to
  be; now it shows the actual HTTP status/reason in the page.
- OccController.php: translated a leftover Dutch comment (missed by
  the earlier Russian->English pass since it wasn't Russian), and
  documented why requireAdmin() is intentional defense-in-depth rather
  than a misunderstanding of the framework: verified against
  Nextcloud's own SecurityMiddleware source that any controller method
  without @NoAdminRequired already rejects non-admins by default, so
  this check (and the original "security fix" commit that added it)
  was addressing a non-issue for THIS controller specifically.
  DbController's equivalent check is still genuinely load-bearing -
  that controller originally shipped with @PublicPage, which does
  bypass the default protection - so it gets a note explaining the
  distinction instead.
- README: added compatibility matrix rows for 0.4.0 (app id rename to
  extended_occweb) and 0.4.1 (these fixes), both verified on NC 34.0.4.

Deferred to when a test instance exists (tracked, not fixed here): no
documented migration path from the old occweb id to extended_occweb
for existing installs, OccController's constructor doing expensive
work before the (redundant) admin check runs, and js/jquery.js loading
unconditionally on NC30-33 where core still provides jQuery.

Version bump to 0.4.1.

js/index.js

@@ -387,8 +387,8 @@
             data: JSON.stringify(occCommand)
           }).done(function (response) {
             term.echo('\n' + response).resume();
-          }).fail(function (response, code) {
-            term.echo('\n' + response).resume();
+          }).fail(function (xhr, status) {
+            term.echo('\n[[;#ff5555;]Request failed: ]' + $.terminal.escape_formatting(xhr.status + ' ' + xhr.statusText)).resume();
           });
         }
       }, {
@@ -410,6 +410,16 @@
           scrollToBottom()
         }
       });
+    }).fail(function (xhr, status) {
+      // Without this, a failure here (expired session/CSRF token, network
+      // error, a rejected admin check) leaves #app-content permanently
+      // empty with no visible error and nothing in the console pointing
+      // at the cause - exactly what happened during NC34 testing before
+      // this handler existed.
+      $('#app-content').text(
+        'Failed to load the terminal: ' + xhr.status + ' ' + xhr.statusText +
+        '. Try reloading the page; if that keeps happening, check the server logs.'
+      );
     });
     $('html').keypress(function(){
       scrollToBottom()