brand: document searchi identity and add reusable logo masters

assets/brand/README.md

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+# Brand assets
+
+The reference board was supplied by Karl. The PNG masters were prepared with
+image generation from that board; they preserve its recognizable S/search motif.
+They are raster assets, not exact vector reconstructions. Review at small sizes
+before a production release; precise flat-color vector masters remain a polish task.
+
+- `searchi-symbol.png`: blue symbol with transparency, used in the app header.
+- `searchi-app.png`: square blue background and white search motif for app/avatar.
+- `reference-board.jpg`: approved supplied brand reference.
+
+Generate platform icons from the same master using the official Tauri CLI:
+
+```sh
+cd apps/desktop
+npm run tauri -- icon ../../assets/brand/searchi-app.png
+```
+
+Desktop icon paths are configured in src-tauri/tauri.conf.json. Generated mobile
+variants are not evidence of mobile product support. The app's public symbol is
+a copy of the master; keep both synchronized when replacing the mark.