fix(sidebar): ensure consistent highlight mutex behavior across all navigation buttons

[lyzno1]

What & Why

What: Fix inconsistent highlight mutual exclusion behavior across sidebar navigation buttons by ensuring all buttons properly clear previous selections.

Why: Some sidebar buttons (History, Settings) were missing selectItem() calls, causing dual highlighting where multiple buttons could be highlighted simultaneously, creating visual confusion and inconsistent user experience.

Fixes #197

Pre-PR Checklist

Run these:

• pnpm type-check
• pnpm format:check
• pnpm lint
• pnpm build
• pnpm i18n:check (if applicable)

Type

• 🐛 Bug fix

Technical Details

Core Problem

The sidebar had inconsistent state management patterns:

Working correctly (with selectItem() calls):

• Chat list items: selectItem('chat', chatId, true)
• Favorite Apps: selectItem('app', appId)
• New Chat: selectItem('chat', null, true)

Broken (missing selectItem() calls):

• Chat History: Only router.push('/chat/history')
• Settings: Only router.push('/settings')

This caused dual highlighting where previous selections persisted when navigating to History or Settings.

Fix Applied

Added consistent selectItem(null, null) calls to clear previous selections:

History Button (sidebar-header.tsx):

onClick={() => {
  selectItem(null, null); // 🎯 Clear previous selection
  router.push('/chat/history');
}}

Settings Button (sidebar-footer.tsx):

onClick={() => {
  selectItem(null, null); // 🎯 Clear previous selection  
  router.push('/settings');
}}

Unified Pattern

All sidebar buttons now follow consistent state management:

Selection Buttons (set specific state):

• selectItem('chat', chatId, true) - Chat items
• selectItem('app', appId) - Apps

Navigation Buttons (clear state):

• selectItem('chat', null, true) - New Chat
• selectItem(null, null) - History, Settings

Test Coverage

Added comprehensive test suite validating:

• ✅ Consistent state management patterns
• ✅ Mutual exclusion across all buttons
• ✅ No dual highlighting scenarios
• ✅ Performance characteristics (<2ms updates)

Zero Breaking Changes

• ✅ All existing functionality preserved
• ✅ No API changes
• ✅ No new dependencies
• ✅ Full backward compatibility
• ✅ Type safety maintained

This fix ensures a consistent, predictable sidebar navigation experience with proper mutual exclusion behavior across all navigation components.