feat(studio): Settings dialog gets macOS-style sidebar categories

The Settings dialog was a single 800-line vertical scroll — Hub
endpoint, channels, System AI, MCP, n8n, registry credentials,
hub auth token, default scope, theme plugin, maintenance all
stacked. Stefan: 'inzwischen zu unübersichtlich als langes
feld, untergruppierungen wären toll, wie z.B. in den MacOS
Einstellungen'.

Splits the panel into six focused categories driven by a
left sidebar:

  - **General**: Hub endpoint (host/port/TLS), channels,
    default scope
  - **Appearance**: theme plugin picker (applies instantly)
  - **System AI**: operator-managed LLM endpoint for Studio
    helpers
  - **Integrations**: MCP servers + n8n endpoints
  - **Security**: registry credentials + hub auth token
  - **Maintenance**: reset / destructive ops

Sidebar uses macOS-style icon + label rows with a highlighted
selection pill. Content area is per-category, scrollable
within its own panel. The dialog widens from 520→820 px and
gains a fixed height (620 px) so the layout doesn't jump as
the operator switches categories.

Each panel opens with a title + one-sentence description so
the operator knows what they're looking at before scanning the
controls — same pattern Apple uses in System Settings.

Also adds the missing 'meta' dependency that FaiLog's
@visibleForTesting needed. Studio bumped to 0.63.0.

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

pubspec.lock

@@ -126,7 +126,7 @@ packages:
       path: "../fai_studio_flow_editor"
       relative: true
     source: path
-    version: "0.15.1"
+    version: "0.16.0"
   fake_async:
     dependency: transitive
     description:
@@ -412,7 +412,7 @@ packages:
     source: hosted
     version: "0.13.0"
   meta:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: meta
       sha256: "1741988757a65eb6b36abe716829688cf01910bbf91c34354ff7ec1c3de2b349"