scaffold Flutter app — kernel, extensions, widgets, Tier 0 built-ins

First real content under app/. Lays the whole Tier 0 foundation in one
commit because the pieces depend on each other circularly (kernel →
extension → widgets → built-ins all reference types from the layer
below); splitting would leave intermediate commits that don't compile.

Key shapes:

  * bare WidgetsApp root — no Material, no Cupertino, no Scaffold.
    ClideTheme InheritedWidget is the only source of tokens.
  * ClideKernel InheritedWidget aggregates 18 services (settings,
    project, extensions, theme, panels, events, ipc, commands +
    palette + keybindings, clipboard, files, notify, dialog, tray,
    secrets, os, net, focus, log, i18n). ExtensionContext exposes
    them through a stable interface.
  * ClideExtension + sealed ContributionPoint hierarchy (Tab,
    StatusItem, Toolbar, Command, TrayItem, LayoutPreset). One
    manifest ships N contributions into kernel slots.
  * Three-tier theme pipeline: palette (named colors) → semantic
    roles → ~60 surface tokens. Defaults at each layer so legacy
    palette-only themes produce a complete SurfaceTokens.
  * A11y baked in from day one — every interactive primitive wraps
    in Semantics(label:, hint:, button:); ensureSemantics() at boot;
    theme/contrast.dart helper exposes token pairs for the WCAG
    gate in the a11y test suite.
  * i18n ported from fframe's L10n pattern (text-driven, namespaced
    JSON catalogs, caller-supplied placeholders) with a proper
    locale fallback chain (exact → language → default → placeholder)
    fframe lacks.
  * Four Tier-0 built-ins live (default-layout, welcome, ipc-status,
    theme-picker) plus 17 id-reserving stubs so later tiers fill in
    without rename churn.

.gitignore extended to cover app/ sub-package artefacts and the
Playwright harness scratch dirs.

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/widgets/src/clide_scrollbar.dart

@@ -0,0 +1,54 @@
+import 'package:clide_app/kernel/src/theme/controller.dart';
+import 'package:flutter/widgets.dart';
+
+/// Thin themed scrollbar. Tier-0 shell; more refined scrolling (velocity
+/// multiplier, keyboard nav) lands with the editor.
+class ClideScrollbar extends StatelessWidget {
+  const ClideScrollbar({
+    super.key,
+    required this.controller,
+    required this.child,
+    this.axis = Axis.vertical,
+  });
+
+  final ScrollController controller;
+  final Widget child;
+  final Axis axis;
+
+  @override
+  Widget build(BuildContext context) {
+    final tokens = ClideTheme.of(context).surface;
+    return ScrollbarTheme(
+      slider: tokens.scrollbarSlider,
+      sliderHover: tokens.scrollbarSliderHover,
+      track: tokens.scrollbarTrack,
+      child: RawScrollbar(
+        controller: controller,
+        thumbColor: tokens.scrollbarSlider,
+        thickness: 8,
+        radius: const Radius.circular(4),
+        child: child,
+      ),
+    );
+  }
+}
+
+class ScrollbarTheme extends InheritedWidget {
+  const ScrollbarTheme({
+    super.key,
+    required this.slider,
+    required this.sliderHover,
+    required this.track,
+    required super.child,
+  });
+
+  final Color slider;
+  final Color sliderHover;
+  final Color track;
+
+  @override
+  bool updateShouldNotify(ScrollbarTheme old) =>
+      slider != old.slider ||
+      sliderHover != old.sliderHover ||
+      track != old.track;
+}