feat: restructure navigation from 5 tabs to 3 tabs

[jaxkodex]

Context

Per specs/design.md, the navigation should be simplified from 5 bottom tabs to 3, reducing clutter and giving more screen real estate to content.

Proposed changes

• Remove Browse and Settings tabs from the bottom navigation bar
• Merge Browse content into a new Library screen with filter tabs: Playlists / Artists / Albums / Songs / Downloads
• Move Settings behind a profile/gear icon in the Home top bar
• New LibraryScreen.kt with filter tab state management

Files affected

• ui/navigation/NavGraph.kt — update BottomNavItem list, routes
• ui/home/HomeScreen.kt — add gear/profile icon to top bar
• ui/home/detail/ — potentially merge into new LibraryScreen.kt
• New: ui/library/LibraryScreen.kt

Flagged from

Design language PR #91