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/pages/defects.vue

@@ -0,0 +1,17 @@
+<script setup lang="ts">
+// Defect-viewing route (`/defects`) — the read-only board with the weak-point
+// map and feed. Same domain as reporting (ADR 0004).
+</script>
+
+<template>
+  <main class="mx-auto flex max-w-5xl flex-col items-center gap-6 p-6">
+    <header class="text-center">
+      <h1>Andon — weak points</h1>
+      <p class="opacity-70">Every defect filed on the board.</p>
+    </header>
+
+    <BoardView />
+
+    <p class="opacity-60">The red-dot map (T7) and defect feed (T8) land next.</p>
+  </main>
+</template>