Simplify navigation by removing unused sidebar mode from both apps:
Todo App:
- Remove isSidebarMode state, handlers, and localStorage persistence
- Remove sidebar-related CSS classes and styles
- Simplify TodoToolbar to pure wrapper component
Contacts App:
- Remove isSidebarMode state, handlers, and localStorage persistence
- Remove sidebar-related CSS from ContactsToolbar and ContactAlphabetView
- Always show view-mode-pill (no longer conditional on sidebar mode)
This removes ~250 lines of unused code across 5 files.
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