test: added test cases for seperator, sheet, sidebar

Description:

This PR adds comprehensive unit test cases for UI components located in src/components/ui:

• separator.tsx (22 tests)
• sheet.tsx (37 tests)
• sidebar.tsx (79 tests)

Separator Tests

• Covers rendering, orientation (horizontal/vertical), styling via cn() utility, and accessibility (role, aria-orientation)
• Tests decorative vs non-decorative modes
• Handles edge cases like empty props, multiple renders, and ref forwarding

Sheet Tests

• Covers rendering, trigger behavior, and visibility (open/close)
• User interactions: click, Escape key, overlay click
• Supports variants (top, bottom, left, right)
• Controlled/uncontrolled behavior with open and onOpenChange
• Accessibility (dialog role, focus handling)
• Portal rendering and async behavior handling

Sidebar Tests

• Covers complex sidebar behavior with context provider
• Tests open/close state, controlled mode, and callbacks
• UI rendering for header, footer, menu, groups, and actions
• Keyboard shortcuts (Ctrl/Cmd + B)
• Mobile responsiveness (sheet-based behavior)
• Accessibility (aria-labels, sr-only text)
• Edge cases: rapid toggling, multiple instances, null props
• Integration with hooks (e.g., useIsMobile) and persistence logic

Key Highlights

• Total: 138 tests
• Uses Jest + React Testing Library
• Real component implementations are tested (minimal mocking)
• External dependencies (hooks, i18n, utilities) are mocked where needed
• Focus on UI behavior, accessibility, and edge cases

---

Type of Change:

• Bug Fix
• Feature Development
• Documentation

---

Testing:

• Tested on desktop
• Tested on mobile
• Tested in different browsers

---

Screenshots (if applicable):

---

Checklist:

• Code follows project style guidelines
• Self-review completed
• No console errors
• TypeScript types are correct

---

closes #280 (closed)