feat(theme): backport wallpaper color extraction to API < 31

[sang765]

Summary

Uses WallpaperManager to extract the device wallpaper bitmap and derive a seed color via the existing Palette-based extractThemeColor(). On API below 31 this seed color feeds into MaterialKolor to produce a full M3 ColorScheme, giving Material You-style wallpaper-adaptive theming on older devices where the system dynamic color API is unavailable.

Changes

• Theme.kt: added extractWallpaperThemeColor(context) function that gets the wallpaper drawable, converts to bitmap, and calls the existing extractThemeColor() Palette extraction
• MainActivity.kt: when no song is playing and dynamic theme is enabled, uses wallpaper color as seed on API below 31 (falls back to customThemeColor if extraction fails)

Behavior

Scenario	API below 31	API 31+
Song playing	Album art color (unchanged)	Album art color (unchanged)
No song, dynamic theme ON	Wallpaper color - new	System dynamic color (unchanged)
No song, dynamic theme OFF	Custom palette/color (unchanged)	Custom palette/color (unchanged)

Notes

• No new dependencies - reuses existing androidx.palette:palette-ktx and com.materialkolor:material-kolor
• WallpaperManager.getDrawable() requires no runtime permissions on API 24+
• Safely falls back to customThemeColor on any extraction failure
• minSdk = 26 ensures WallpaperManager.getDrawable() (API 24) is always available