Terminal-Browser: nicht erreichbare Startseite wird vorab gemeldet

Symptom war ein scheinbar haengender Browsh: Logo, "Waiting for Firefox
to connect...", danach schwarz. Ursache ist nicht der Browser - Browsh
laeuft (Strg+Q kehrt ins Menue zurueck) - sondern die Startseite. Auf
http://127.0.0.1:81 antwortet nichts, solange der Proxy-Manager-Container
nicht laeuft, und ein Terminal-Browser zeigt dann keine Fehlerseite,
sondern gar nichts.

cmd_browser prueft die Startseite deshalb vorher und nennt die Ursache
statt stumm zu starten. Unterschieden werden: Proxy Manager
abgeschaltet (NPM_ENABLED=off), Container fehlt, Container gestoppt,
Container laeuft aber antwortet nicht, tote Checkmk-Oberflaeche auf
Port 5000 und externe Adressen. Jeweils mit dem passenden naechsten
Kommando. Ueberspringen mit --no-check; in der Oberflaeche erscheint
dieselbe Diagnose als Dialog mit der Wahl, trotzdem zu starten.

Dabei gefunden und behoben - an drei Stellen dieselbe Falle:
curl gibt mit -w '%{http_code}' im Fehlerfall selbst schon "000" aus.
Das zusaetzliche "|| echo 000" haengte einen zweiten Wert an, sodass
der Vergleich nie zutraf. Betroffen waren die neue Erreichbarkeits-
pruefung sowie http_code() und die Registry-Erreichbarkeit im
Zugangsdaten-Test ("image test") - dort waere ein nicht erreichbarer
Endpunkt als "unerwartete Antwort" statt als "nicht erreichbar"
gemeldet worden. Jetzt wird der Fehler der Zuweisung abgefangen.

Alle vier Diagnosezweige im Terminal durchgespielt, der
Zugangsdaten-Test zur Gegenprobe erneut gegen Docker Hub gelaufen.
Dokumentiert als Abschnitt 11.3 in beiden Sprachfassungen.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README_en.md

@@ -1014,7 +1014,43 @@ so that Podman does not look for them at `registry.access.redhat.com`.
 In the ncurses interface everything lives under *"Terminal-Browser (Proxy
 Manager lokal bedienen)"*.
 
-### 11.3 Limitation: file upload
+### 11.3 Blank screen: the start page does not answer
+
+If everything stays black after `Starting Browsh …` /
+`Waiting for Firefox to connect…`, it is almost always because **nothing
+answers on the start page** – not because of the browser. A terminal browser
+shows no error page in that case, just nothing. You can tell that Browsh itself
+is running because `Ctrl+Q` still returns you to the menu.
+
+The script therefore checks the start page beforehand and names the cause:
+
+```
+[ WARN ] Auf http://127.0.0.1:81 antwortet nichts.
+Ein Terminal-Browser zeigt dann nur eine schwarze Seite.
+
+Das ist die Oberflaeche des NGINX Proxy Managers.
+Er ist abgeschaltet (NPM_ENABLED=off):
+    ./scripts/cmk-manage.sh npm-enable on
+
+Trotzdem starten? [j/N]
+```
+
+("nothing answers on …", "a terminal browser then shows only a black page",
+"it is switched off", "start anyway?") The cases it distinguishes:
+
+| Situation | Message |
+|---|---|
+| Proxy Manager switched off | points to `npm-enable on` |
+| container missing or stopped | points to `up` |
+| container runs but does not answer | points to `logs` and `npm-port` |
+| Checkmk interface (port 5000) dead | points to `up` or `sites` |
+| external address | points to the host's network access |
+
+`--no-check` skips the check. In the interface the same diagnosis appears as a
+dialog offering to start anyway or to pick a different start page from the
+menu.
+
+### 11.4 Limitation: file upload
 
 A terminal browser cannot open the operating system's file chooser. For
 **Let's Encrypt** that does not matter (only form fields are filled in), but
@@ -1073,7 +1109,7 @@ sudo ./scripts/cmk-manage.sh browser --engine shell
 > address and password. That is easiest in the browser
 > (`cmk-manage.sh browser`); afterwards the API works with the new credentials.
 
-### 11.4 Alternative: SSH tunnel
+### 11.5 Alternative: SSH tunnel
 
 If SSH access to the machine is possible, a tunnel is the most convenient
 route – you then use the interface in your usual browser:
@@ -1843,8 +1879,9 @@ release notes beforehand.
 | Admin interface unreachable from the LAN | port 81 blocked. Move it to a free port: `cmk-manage.sh npm-port 10443` and open that in the firewall |
 | New port does not answer after `npm-port` | the Proxy Manager was not restarted: `cmk-manage.sh restart`; check the file `data/npm/data/nginx/custom/http.conf` |
 | `cmk-manage.sh` aborts with "CMK_RUNTIME … ist nicht installiert" | it now only warns and detects the runtime itself; set it cleanly with `config set CMK_RUNTIME auto` |
+| Browser stays black, "Waiting for Firefox to connect…" | The start page does not answer – usually the Proxy Manager is not running. The script now reports this beforehand, see [chapter 11.3](#113-blank-screen-the-start-page-does-not-answer) |
 | Proxy Manager interface blank / only text fragments | a text browser without JavaScript was used. Use `cmk-manage.sh browser --engine browsh` (or `carbonyl`) |
-| Self-signed certificate cannot be uploaded in the terminal browser | there are no file dialogs there – upload via the API, see [chapter 11.3](#113-limitation-file-upload) |
+| Self-signed certificate cannot be uploaded in the terminal browser | there are no file dialogs there – upload via the API, see [chapter 11.4](#114-limitation-file-upload) |
 | `browser` reports "braucht ein echtes Terminal" (needs a real terminal) | called from a pipe/script. Call it on the console or over SSH; only the build runs headless, with `--build-only` |
 | A menu entry drops out to the shell with an error | fixed: missing containers, sites or backups are now reported in a dialog and the interface stays open |
 | `job auto on` reports missing permissions | `/etc/cron.d/` needs root: call it with `sudo`, or add the printed lines yourself |