M0: Vue 3 + TypeScript frontend — auth views + authed board shell

- Vite + Vue 3.5 + Pinia + vue-router + Tailwind (brand accent #F5C518),
  dark-mode aware, deterministic package-lock.json for `npm ci`.
- Session store (fetchMe/login/register/logout) over a credentials:'include'
  fetch client; router guards (requiresAuth / guestOnly) with lazy /me resolve.
- BaseButton + BaseInput primitives (focus rings, loading, error states).
- LoginView, RegisterView, and an authed BoardView shell with an empty state
  for the M1 masonry board — all at v1 polish (rule 24).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/style.css

@@ -0,0 +1,19 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+html,
+body,
+#app {
+  height: 100%;
+}
+
+body {
+  @apply bg-neutral-50 text-neutral-900 antialiased;
+}
+
+@media (prefers-color-scheme: dark) {
+  body {
+    @apply bg-neutral-950 text-neutral-100;
+  }
+}