feat: add default icon folder guide

Author: SayanthRock

themes/default/icons/README.md

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+# Icons
+
+Add icon preview assets here.
+
+Recommended files:
+
+- icon-preview.png
+- icon-grid-preview.png
+- adaptive-icon-sample.png
+
+## Style guide
+
+- Rounded shapes
+- Simple symbols
+- Good contrast on dark background
+- Desert Sand accent highlights
+- Consistent spacing
+
+## Customer note
+
+Icon support depends on launcher and OEM theme support. Test on each device model before marking stable.