add 1px breathing room above tab strips (T-324)

The tab strips — Claude session tabs (MultitabPane._TabStrip) and the
slot tab bar (ClideTabBar) — sat flush against the chrome above, reading
as cramped. Add a 1px top margin to both Containers so the pane surface
behind shows as a hairline gap, separating the tabBarBackground strip
from the chrome edge. Both are single-use widgets, so the margin applies
once per surface (no double-apply).

Regenerated the linux clide_tab_bar golden. (The macOS variant is now 1px
stale but is dormant on the linux gate; regenerate on macOS.)

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

lib/widgets/src/clide_tab_bar.dart

@@ -44,6 +44,9 @@ class ClideTabBar extends StatelessWidget {
       explicitChildNodes: true,
       child: Container(
         height: height,
+        // A hairline of the surface behind, so the strip doesn't butt against
+        // the chrome directly above it (T-324).
+        margin: const EdgeInsets.only(top: 1),
         color: tokens.tabBarBackground,
         child: Row(
           children: [

lib/widgets/src/multitab_pane.dart

@@ -120,6 +120,9 @@ class _TabStrip<T> extends StatelessWidget {
 
     return Container(
       height: tabHeight,
+      // A hairline of the surface behind, so the strip doesn't butt against the
+      // chrome directly above it (T-324).
+      margin: const EdgeInsets.only(top: 1),
       decoration: BoxDecoration(
         color: tokens.tabBarBackground,
         border: Border(bottom: BorderSide(color: tokens.dividerColor)),