fix(ui): hide duplicate close button in workspace header at mobile width (#783)

At the @media(max-width:900px) breakpoint both .close-preview and .mobile-close-btn were visible simultaneously. Since boot.js wires both to handleWorkspaceClose(), only the mobile-close-btn needs to show at that width. Adds .close-preview{display:none} to the 900px media block.

Fixes #781

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.122] — 2026-04-21
+
+### Fixed
+- **Duplicate X button in workspace panel header on mobile** — at viewport widths ≤900px the desktop close-preview button (`.close-preview` / `btnClearPreview`) is now hidden via CSS, leaving only the mobile close button (`.mobile-close-btn`) visible. Previously both buttons appeared side-by-side when the window was resized below the 900px breakpoint. (#781)
+
 ## [v0.50.121] — 2026-04-20
 
 ### Performance