feat(branding): add @brand vite alias + migrate in-app img consumers

vite-branding.ts is the shared resolver. Exports BRAND_DIR (absolute,
defaults to ./branding/default) and brandAlias for spreading into each
vite config's resolve.alias map.

All 9 vite configs now spread brandAlias so `@brand/<file>` resolves
to the active brand dir at build time.

Migrates the four <img src="@/assets/logo.png"> consumers
(Login.vue, LoginDemo.vue, AppSidebar.vue, MobileDrawer.vue) to
@brand/logo.png. Unused Navbar.old.vue left as-is.

Build verified: dist/assets/logo-<hash>.png emits from the aliased
import. Future deployers point BRAND_DIR at their brand kit and the
in-app logo follows automatically.

Part of aiolabs/webapp#95.

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

vite-branding.ts

@@ -0,0 +1,19 @@
+import { resolve } from 'node:path'
+
+/**
+ * Absolute path to the active brand kit. Deployers point this at their
+ * own `branding/<name>/` directory (see branding/README.md).
+ *
+ * Defaults to the committed aiolabs default brand. Used by vite configs
+ * for the `@brand` import alias and by pwa-assets.config.ts.
+ */
+export const BRAND_DIR = resolve(process.env.BRAND_DIR ?? './branding/default')
+
+/**
+ * Spread into a vite config's `resolve.alias` map. Lets components
+ * import deployer-provided assets via `@brand/<file>` instead of
+ * hardcoding `@/assets/logo.png`.
+ */
+export const brandAlias = {
+  '@brand': BRAND_DIR,
+} as const