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>

CHANGELOG.md

@@ -256,6 +256,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- **Tab strips get a hairline of breathing room.** The tab strip (Claude session
+  tabs, slot tabs) butted flush against the chrome above it, reading as cramped;
+  it now sits 1px below, the pane surface showing through the gap. (T-324)
 - **Re-showing an image after it changes on disk now refreshes.** Image cards,
   thumbnails, the lightbox, and `clide image show` keyed Flutter's image cache by
   path alone, so overwriting a file in place (e.g. re-exporting a wireframe PNG)