Flatten desktop layout and stabilize sidebar splits

[oxwen11]

Summary

• Remove the AppShell wrappers and redundant div containers so Topbar and SecondarySidebar remain outside SplitRoot, and the desktop layout flows through two nested splitters.
• Keep PrimarySidebarOverlay behavior intact while sinking both splitters directly into the main render tree for desktop and narrow modes.
• Let the primary sidebar fill its parent and enforce the new splitter size defaults/min/max so the primary pane stays wider and drag handles reflect its actual width.

Testing

• Not run (not requested)