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.libra.config.ts

@@ -5,6 +5,7 @@ import { defineConfig, type Plugin } from 'vite'
 import { VitePWA } from 'vite-plugin-pwa'
 import { ViteImageOptimizer } from 'vite-plugin-image-optimizer'
 import { visualizer } from 'rollup-plugin-visualizer'
+import { brandAlias } from './vite-branding'
 
 /**
  * Plugin to rewrite dev server requests to libra.html
@@ -110,6 +111,7 @@ export default defineConfig(({ mode }) => ({
   ],
   resolve: {
     alias: {
+      ...brandAlias,
       // ORDER MATTERS — @rollup/plugin-alias is first-match-wins.
       // The more specific @/app.config remap must precede the @ prefix
       // alias, otherwise '@/app.config' matches '@' first and resolves