Fix collapsed accordion taking 100% of viewport height instead of container height

Resolves issue where collapsed channels were sizing to 100% of screen height
rather than 100% of the actual window container. Replaced explicit height: 100%
with flex: 1 to let flexbox naturally constrain the collapsed channel height
to match its siblings in the flex row.

Changes:
- Removed height: 100% and max-height: 100% from .channel-column--collapsed .channel-subheader
- Added flex: 1 to fill available space within the natural flex container bounds
- Collapsed channels now properly size to their row height, not viewport height

This ensures collapsed channels stay within the mosaic window boundaries
and don't extend beyond the visible area.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MarkoVcode

benchmesh-serial-service/frontend/src/ui/workbench/EditorArea/editor.css

@@ -258,8 +258,7 @@
 .channel-column--collapsed .channel-subheader {
   writing-mode: vertical-lr;
   transform: rotate(180deg);
-  height: 100%;
-  max-height: 100%;
+  flex: 1;
   width: 100%;
   border-bottom: none;
   border-right: 1px solid var(--editor-border);