feat: add default preview folder guide

Author: SayanthRock

themes/default/previews/README.md

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+# Previews
+
+Add customer preview images here.
+
+Recommended files:
+
+- cover.png
+- before-home.png
+- after-home.png
+- before-lock.png
+- after-lock.png
+- feature-banner.png
+
+## Preview rules
+
+- Show real device screenshots when possible
+- Keep images clean and readable
+- Do not overpromise features
+- Mark device model if preview is device-specific