[client] Mark Rosenpass as deprecated (#8372)

* [client] Mark Rosenpass flags as deprecated

Rosenpass support is being deprecated. Users enabling it from the CLI
should learn that before they depend on it, while the feature itself
keeps working.

* [client] Show Rosenpass deprecation notice in the UI settings

Desktop users enable Rosenpass from the settings toggle, not the CLI,
so the deprecation notice needs to reach them there too. A new string
key falls back to English until Crowdin translates it, instead of
showing a stale translation without the notice.

* [client] Keep switchOrCreateProfile doc comment attached

The Rosenpass warning helper landed between switchOrCreateProfile and
its doc comment, leaving the function undocumented.

* [client] Use MarkDeprecated for the Rosenpass flags

Follows the convention already used for --enable-lazy-connection, as
asked in review. The flags drop out of --help and pflag warns on any
use, so the custom helper is no longer needed.

* [client] Isolate the Rosenpass flag test from upCmd's flag set

Setting the shared upCmd flags left them marked Changed for any later
test in the package.

client/ui/i18n/locales/en/common.json

@@ -899,6 +899,10 @@
         "message": "Add a post-quantum key exchange via Rosenpass on top of WireGuard®.",
         "description": "Helper text: adds a post-quantum key exchange via Rosenpass on top of WireGuard®. 'Rosenpass' and 'WireGuard®' are product names — do not translate; keep the ® symbol."
     },
+    "settings.security.rosenpass.deprecated": {
+        "message": "Deprecated: Rosenpass support may be removed in a future release.",
+        "description": "Notice shown under the quantum-resistance toggle: the Rosenpass feature is deprecated and may be removed later. 'Rosenpass' is a product name — do not translate."
+    },
     "settings.security.rosenpassPermissive.label": {
         "message": "Enable Permissive Mode",
         "description": "Toggle label: enable permissive mode (for quantum-resistance)."