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_button.dart

@@ -0,0 +1,105 @@
+import 'package:clide_app/kernel/src/theme/controller.dart';
+import 'package:clide_app/widgets/src/clide_text.dart';
+import 'package:flutter/widgets.dart';
+
+enum ClideButtonVariant { normal, primary, subtle }
+
+class ClideButton extends StatefulWidget {
+  const ClideButton({
+    super.key,
+    required this.label,
+    required this.onPressed,
+    this.variant = ClideButtonVariant.normal,
+    this.padding = const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
+    this.semanticLabel,
+    this.semanticHint,
+  });
+
+  final String label;
+  final VoidCallback? onPressed;
+  final ClideButtonVariant variant;
+  final EdgeInsetsGeometry padding;
+
+  /// Overrides [label] for screen readers (use when the visible label is
+  /// an icon-only glyph or a noun that reads oddly when announced).
+  final String? semanticLabel;
+
+  /// Screen-reader hint describing the button's effect. Optional.
+  final String? semanticHint;
+
+  @override
+  State<ClideButton> createState() => _ClideButtonState();
+}
+
+class _ClideButtonState extends State<ClideButton> {
+  bool _hovered = false;
+  bool _pressed = false;
+
+  @override
+  Widget build(BuildContext context) {
+    final tokens = ClideTheme.of(context).surface;
+    final enabled = widget.onPressed != null;
+
+    Color bg;
+    Color fg;
+    switch (widget.variant) {
+      case ClideButtonVariant.normal:
+        bg = _pressed
+            ? tokens.buttonActiveBackground
+            : _hovered
+                ? tokens.buttonHoverBackground
+                : tokens.buttonBackground;
+        fg = tokens.buttonForeground;
+      case ClideButtonVariant.primary:
+        bg = _pressed
+            ? tokens.panelActiveBorder
+            : _hovered
+                ? tokens.buttonActiveBackground
+                : tokens.buttonActiveBackground;
+        fg = tokens.globalBackground;
+      case ClideButtonVariant.subtle:
+        bg = _hovered
+            ? tokens.listItemHoverBackground
+            : tokens.listItemBackground;
+        fg = tokens.listItemForeground;
+    }
+
+    return Semantics(
+      button: true,
+      enabled: enabled,
+      label: widget.semanticLabel ?? widget.label,
+      hint: widget.semanticHint,
+      onTap: enabled ? widget.onPressed : null,
+      excludeSemantics: true,
+      child: MouseRegion(
+        cursor:
+            enabled ? SystemMouseCursors.click : SystemMouseCursors.forbidden,
+        onEnter: (_) => setState(() => _hovered = true),
+        onExit: (_) => setState(() => _hovered = false),
+        child: GestureDetector(
+          onTapDown: (_) => setState(() => _pressed = true),
+          onTapCancel: () => setState(() => _pressed = false),
+          onTapUp: (_) {
+            setState(() => _pressed = false);
+            widget.onPressed?.call();
+          },
+          child: Container(
+            padding: widget.padding,
+            decoration: BoxDecoration(
+              color: bg,
+              border: Border.all(color: tokens.buttonBorder, width: 1),
+              borderRadius: BorderRadius.circular(3),
+            ),
+            child: ClideText(
+              widget.label,
+              color: fg,
+              fontWeight: widget.variant == ClideButtonVariant.primary
+                  ? FontWeight.w600
+                  : FontWeight.w500,
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}