Remove sidebar rail toggle

[touch2be]

Summary

• remove the sidebar edge rail toggle so the resize-like hover target no longer appears
• keep the title-bar sidebar toggle intact for manual collapsing

Verification

• git diff --check
• vp check src/components/AppSidebar.tsx could not run in the clean worktree because dependencies are not installed there; the same affected files were checked successfully in the original workspace before creating this isolated branch