fix(design-systems): the empty state's create button did nothing, and one surface

Two reports, one root cause each.

**The dead button.** "Create the first one" set `showCreate = true`, but the
create form lived inside `<div v-else class="ds-body">` — the sibling branch of
the empty state. The two are mutually exclusive, so on a fresh install the flag
flipped and nothing rendered. The first action a new install can take was the
one that didn't work, which is a poor way to honour "an install with zero design
systems is the ordinary state".

The first system now gets its own form outside the list layout, and it drops the
parent picker entirely: there is nothing to inherit from yet, so it says so
instead of offering an empty select.

**Two surfaces, the wrong one first.** /design and /design-systems are halves of
one thing — the record that decides the styling, and what the browser renders
from it — and I had added them as two separate nav entries with the read-only
diagnostic listed first. Backwards: the record is what you work with; the live
view is the check on it.

Now one nav entry pointing at the record, with a shared tab bar joining the two.
The explorer is renamed "Live tokens", which is what it actually shows.

The tab bar is a component rather than the same markup in both views. Two copies
diverge the moment a third tab appears — and a design surface that ships
duplicated markup would be arguing against itself.

frontend/src/components/AppHeader.vue

@@ -64,10 +64,13 @@ router.afterEach(() => {
           <Moon v-else :size="16" />
         </button>
 
-        <!-- Design explorer. An icon rather than a sixth primary nav link: it's a
+        <!-- Design. An icon rather than a sixth primary nav link: it's a
              meta-surface like Trash and Settings, but hiding it entirely would
-             defeat the point of having somewhere the design system is visible. -->
-        <router-link to="/design" class="btn-icon" aria-label="Design" title="Design">
+             defeat the point of having somewhere the design system is visible.
+             Points at the RECORD, not the live-token view — the record is what
+             you work with; the live view is the check on it, and it's a tab
+             away. -->
+        <router-link to="/design-systems" class="btn-icon" aria-label="Design" title="Design">
           <Palette :size="16" />
         </router-link>
 
@@ -105,8 +108,7 @@ router.afterEach(() => {
       <router-link to="/rules" class="nav-link">Rulebooks</router-link>
       <router-link to="/shared" class="nav-link">Shared</router-link>
       <div class="mobile-divider"></div>
-      <router-link to="/design" class="nav-link">Design</router-link>
-      <router-link to="/design-systems" class="nav-link">Design systems</router-link>
+      <router-link to="/design-systems" class="nav-link">Design</router-link>
       <router-link to="/trash" class="nav-link">Trash</router-link>
       <router-link to="/settings" class="nav-link">Settings</router-link>
       <div class="mobile-divider"></div>

frontend/src/components/DesignTabs.vue

@@ -0,0 +1,51 @@
+<script setup lang="ts">
+/**
+ * Sub-navigation for the Design surface.
+ *
+ * There are two pages here and they are halves of ONE thing: the record that
+ * decides the styling, and what the browser is actually rendering from it. They
+ * were briefly two top-level nav entries, which put the read-only diagnostic
+ * first and buried the editable record under it — backwards, since the record
+ * is the thing you work with and the live view is the check on it.
+ *
+ * A component rather than the same markup pasted into both views: two copies of
+ * a tab bar diverge the moment a third tab appears, and that is the exact shape
+ * of duplication this whole surface exists to make visible.
+ */
+</script>
+
+<template>
+  <nav class="design-tabs" aria-label="Design views">
+    <router-link to="/design-systems" class="design-tab">Design system</router-link>
+    <router-link to="/design" class="design-tab">Live tokens</router-link>
+  </nav>
+</template>
+
+<style scoped>
+.design-tabs {
+  display: flex;
+  gap: 0.25rem;
+  margin-bottom: 1.25rem;
+  border-bottom: 1px solid var(--color-border);
+}
+
+.design-tab {
+  padding: 0.5rem 0.9rem;
+  font-size: 0.9rem;
+  color: var(--color-text-secondary);
+  text-decoration: none;
+  border-bottom: 2px solid transparent;
+  margin-bottom: -1px;
+}
+
+.design-tab:hover {
+  color: var(--color-text);
+}
+
+/* `router-link-active` rather than `-exact-active`: both routes are leaves, and
+   exact matching would drop the highlight on any future child route. */
+.design-tab.router-link-active {
+  color: var(--color-primary);
+  border-bottom-color: var(--color-primary);
+}
+</style>