feat(branding): scaffold default brand kit

Introduces branding/default/ as the unparameterized aiolabs brand:
- logo.png (1024x1024, seeded from src/assets/logo.png)
- brand.json with name, shortName, themeColor, backgroundColor

First step toward white-label PWA branding (aiolabs/webapp#95). No
consumer wiring yet — that's the next commits. Future deployers
(NixOS hosts in server-deploy, third-party white-labelers) point
BRAND_DIR at their own variant of this layout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

branding/default/brand.json

@@ -0,0 +1,6 @@
+{
+  "name": "AIO",
+  "shortName": "AIO",
+  "themeColor": "#ffffff",
+  "backgroundColor": "#ffffff"
+}