feat: equalizer screen with 5-band sliders and presets

[jaxkodex]

Context

Per specs/design.md, the app should include an equalizer screen with a retro LED-style aesthetic matching the Winamp-inspired design language.

Proposed changes

• New EqualizerScreen.kt with:
  • 5-band vertical sliders (60Hz, 230Hz, 910Hz, 3.6kHz, 14kHz)
  • Gain range: -12dB to +12dB per band
  • Preset buttons: Flat, Bass Boost, Rock, Pop, Classical, etc.
  • JetBrains Mono labels for frequency values and dB readouts (retro LED display feel)
  • AccentPrimary active slider track color
• Backend: Android Equalizer audio effect API, attached to the active AudioSession
• Access point: button in NowPlayingScreen or Settings

Files affected

• New: ui/equalizer/EqualizerScreen.kt
• New: ui/equalizer/EqualizerViewModel.kt
• data/playback/PlaybackManager.kt — expose audioSessionId
• ui/navigation/NavGraph.kt — add Equalizer route

Flagged from

Design language PR #91