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.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.62.1
+version: 0.63.0
 
 environment:
   sdk: ^3.11.0-200.1.beta
@@ -13,6 +13,8 @@ dependencies:
     sdk: flutter
   cupertino_icons: ^1.0.8
   intl: any
+  # `@visibleForTesting` annotation on FaiLog's test seam.
+  meta: any
 
   # Sibling package; will move to Forgejo path once published.
   fai_client_sdk: