dissolve app/ into repo root (D-056)

Single Flutter package at the repo root. All code, tests, assets,
and platform directories moved from app/ to root. Package renamed
from clide_app to clide — all imports rewritten. Merged pubspec
combines core (ffi) and app (flutter, yaml, xterm) dependencies.
Makefile simplified: no APP_PRESENT conditionals, no cd, no daemon
lifecycle. 317 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_scrollbar.dart

@@ -0,0 +1,54 @@
+import 'package:clide/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;
+}