feat(settings): Extensions tab — "watch this space" notice (T-456)

Built-in extensions are always on and there's no third-party install path yet,
so the Extensions tab is a notice rather than a toggle list — a toggle list
would let users brick the app by disabling load-bearing builtins. It explains
that installing/enabling/disabling arrives with third-party (Lua) extensions
and points at the records that pick it up (D-16 / T-8).

Fleshes out the builtin.extensions-ui stub via the custom-control hatch. Adds
two renderer guards — empty section/field labels render no chrome — so a
notice-only category shows just its card.

Tests: contributes the Extensions category + notice control; the notice renders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/settings_ui/src/settings_category_view.dart

@@ -125,10 +125,11 @@ class _SectionCard extends StatelessWidget {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: [
-          Padding(
-            padding: const EdgeInsets.only(left: 2, bottom: 6),
-            child: ClideText(section.label.toUpperCase(), fontSize: clideFontCaption, color: tokens.sidebarSectionHeader, fontFamily: clideMonoFamily),
-          ),
+          if (section.label.isNotEmpty)
+            Padding(
+              padding: const EdgeInsets.only(left: 2, bottom: 6),
+              child: ClideText(section.label.toUpperCase(), fontSize: clideFontCaption, color: tokens.sidebarSectionHeader, fontFamily: clideMonoFamily),
+            ),
           ClideSurface(
             // Card surface (surface.md): panelHeader resolves to the `surface`
             // palette key (the elevated card tone); inputs inside recede to
@@ -181,8 +182,7 @@ class _FieldRow extends StatelessWidget {
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.stretch,
           children: [
-            label,
-            const SizedBox(height: 10),
+            if (field.label.isNotEmpty) ...[label, const SizedBox(height: 10)],
             builder?.call(context) ?? const SizedBox.shrink(),
           ],
         ),