feat(studio): registry credentials panel in Settings (v0.44.0)

The hub install path now reads `~/.fai/registry-token` as a
fallback when `FAI_REGISTRY_TOKEN` is unset (platform v0.10.92).
Studio now writes that file directly: a fresh operator pastes
the PAT into Settings → Registry credentials, hits Save, and
the next install attempt resolves the auth wall without any
shell or env-var setup.

The token never round-trips back into Studio after save —
status is shown only as "Configured (40 chars)" / "Not set"
with no display of the secret itself. The Clear action deletes
the file. On Unix the file is chmod-ed to 0600 (owner-only
read/write); Windows leaves the default user ACL in place.

Storage is strictly local: `~/.fai/registry-token`, never sent
to a remote service. The hint text under the field says so to
make the data flow explicit.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.43.0
+version: 0.44.0
 
 environment:
   sdk: ^3.11.0-200.1.beta