feat: add alpha preview brand badge

docs/TASKS/app-shell/003-add-alpha-preview-brand-badge.md

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+# Task: Add alpha preview brand badge
+
+## Goal
+
+Add a compact product-stage badge beside the authenticated app brand and public landing brand so users can see the product is in alpha preview without treating it as an error or blocking alert. Remove the old authenticated sidebar caption so the brand treatment stays focused.
+
+## Relevant Files
+
+- `frontend/src/layouts/main/AppSidebar.vue`
+- `frontend/src/static/components/LandingSiteMenu.vue`
+- `frontend/src/locales/en.json`
+- `frontend/src/locales/fr.json`
+
+## Validation
+
+```bash
+cd frontend
+npm run build
+```