Fix channel list scroll interruption (#5815)

## Summary
- keep transparent channel-list gaps in Flutter's gesture arena
- allow a new drag to interrupt active ballistic scrolling immediately
- add a behavioral fling-and-counter-drag regression test

## Scope audit
- audited mobile list and scroll constructors across `mobile/lib`
- channels is the only app scrollable overriding `hitTestBehavior`
- all other lists retain Flutter's default opaque hit testing and do not
share this defect

## Verification
- regression test fails before the production change: ballistic offset
continues from `271.17` to `345.56`
- focused interruption regression passes with the fix
- profile/community control test passes
- pre-commit: Dart formatting and Flutter analyzer pass
- pre-push: complete mobile suite passes, 1323 tests
- simulator: immediate counter-drag from the transparent gutter
interrupts deceleration

Simulator evidence:
`/Users/wesb/.buzz/.scratch/mobile-scroll-videos/interruption-verified.mp4`

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

mobile/lib/features/channels/channels_page/body.dart

@@ -45,9 +45,10 @@ class _ChannelsBody extends StatelessWidget {
             onRefresh: onRefresh,
             child: CustomScrollView(
               controller: scrollController,
-              // The transparent gap shows the top section and must not absorb
-              // taps meant for the community or profile controls beneath it.
-              hitTestBehavior: HitTestBehavior.deferToChild,
+              // Transparent list gaps must remain hit-testable so a new drag
+              // can interrupt ballistic scrolling. The app bar is painted
+              // later and retains its community and profile controls.
+              hitTestBehavior: HitTestBehavior.translucent,
               slivers: [
                 SliverToBoxAdapter(child: SizedBox(height: barHeight)),
                 if (usesPinnedGradient)

mobile/test/features/channels/channels_page_test.dart

@@ -232,6 +232,55 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  testWidgets('interrupts a ballistic scroll from a transparent list gap', (
+    tester,
+  ) async {
+    tester.view.physicalSize = const Size(320, 480);
+    tester.view.devicePixelRatio = 1;
+    addTearDown(tester.view.reset);
+    final channels = List.generate(
+      40,
+      (index) => Channel(
+        id: 'channel-$index',
+        name: 'channel-$index',
+        channelType: 'stream',
+        visibility: 'open',
+        description: 'Channel $index',
+        createdBy: 'abc',
+        createdAt: DateTime(2025),
+        memberCount: 10,
+        isMember: true,
+      ),
+    );
+    await tester.pumpWidget(
+      buildTestable(
+        overrides: [
+          channelsProvider.overrideWith(() => _FakeNotifier(channels)),
+        ],
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    final scrollView = find.byType(CustomScrollView);
+    final scrollable = tester.state<ScrollableState>(
+      find.descendant(of: scrollView, matching: find.byType(Scrollable)).first,
+    );
+    await tester.fling(scrollView, const Offset(0, -300), 2400);
+    await tester.pump(const Duration(milliseconds: 32));
+    final ballisticOffset = scrollable.position.pixels;
+    await tester.pump(const Duration(milliseconds: 32));
+    expect(scrollable.position.pixels, greaterThan(ballisticOffset));
+
+    // x=1 is inside the scroll viewport but outside the padded section rows.
+    // A drag beginning here must still enter the scrollable's gesture arena.
+    final interruptingDrag = await tester.startGesture(const Offset(1, 300));
+    await interruptingDrag.moveBy(const Offset(0, 80));
+    await tester.pump();
+
+    expect(scrollable.position.pixels, lessThan(ballisticOffset));
+    await interruptingDrag.up();
+  });
+
   testWidgets('keeps the last channel above the floating tab bar', (
     tester,
   ) async {