feat: add default wallpaper folder guide

Author: SayanthRock

themes/default/wallpapers/README.md

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+# Wallpapers
+
+Add default theme wallpapers here.
+
+Recommended files:
+
+- home-default.jpg
+- lock-default.jpg
+- preview-home.jpg
+- preview-lock.jpg
+
+## Style guide
+
+- Dark premium background
+- Desert Sand accent glow
+- Clean subject area
+- No text inside wallpaper unless needed
+- Keep file size reasonable for mobile users
+
+## Customer note
+
+Some phones allow separate home and lock screen wallpaper. Some phones show only one wallpaper option depending on ROM version.