feat: album-aware theme tinting and full-screen background

[joshhbk]

Summary

• Theme colors now blend with album art's dominant color instead of replacing it entirely
• Removed the rectangular "now playing" panel - full-screen tinted background with subtle grain
• Swipe gesture still cycles through themes, but each theme "tints" the album color rather than overriding it

Changes

• ColorBlending.swift (new): HSB color utilities for hue interpolation, luminance calculation
• TintedThemeProvider.swift (new): Computes blended colors from album + theme
• Shaders.metal: Simplified to subtle grain texture (removed directional brushing)
• BrushedMetalBackground.swift: Now reads theme from environment, full-screen coverage
• ClassicPlayerLayout.swift: Removed ShuffleBodyView wrapper, content floats directly
• PlayerView.swift: Integrated TintedThemeProvider, uses new background

Test plan

• Play songs with different album art colors
• Swipe left/right to change themes - color should shift while staying harmonious with album
• Verify text readability on light and dark blended colors
• Check empty state (no songs) falls back to pure theme color
• Device tilt should not cause visible artifacts on background

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