refactor(app): serve one domain — /defects route and header nav

Drop the two-subdomain split (ADR 0004): report at /, view defects at
/defects, with a shared layout + header nav. Renames the dashboard page to
defects.vue and updates plan/README. Transparency over separation now that
auth gates everyone, and it removes the Coolify two-domain routing question.

app/layouts/default.vue

@@ -0,0 +1,26 @@
+<script setup lang="ts">
+// Single domain (ADR 0004): report at `/`, view all defects at `/defects`,
+// with a persistent header to move between them.
+</script>
+
+<template>
+  <div class="min-h-screen">
+    <header class="navbar border-base-300 border-b">
+      <NuxtLink to="/" class="btn btn-ghost text-xl">Andon</NuxtLink>
+      <nav class="ml-auto flex gap-1">
+        <NuxtLink to="/" class="btn btn-ghost btn-sm" exact-active-class="btn-active">
+          Report
+        </NuxtLink>
+        <NuxtLink
+          to="/defects"
+          class="btn btn-ghost btn-sm"
+          exact-active-class="btn-active"
+        >
+          View defects
+        </NuxtLink>
+      </nav>
+    </header>
+
+    <slot />
+  </div>
+</template>