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/views/DesignView.vue

@@ -21,6 +21,7 @@
 import { computed, onMounted, ref } from "vue";
 
 import { fetchDesignExpectations } from "@/api/design";
+import DesignTabs from "@/components/DesignTabs.vue";
 import PriorityBadge from "@/components/PriorityBadge.vue";
 import StatusBadge from "@/components/StatusBadge.vue";
 import TagPill from "@/components/TagPill.vue";
@@ -101,8 +102,10 @@ const TYPE_SPECIMENS = [
 
 <template>
   <div class="design-view">
+    <DesignTabs />
+
     <header class="design-header">
-      <h1>Design</h1>
+      <h1>Live tokens</h1>
       <p class="lede">
         The system as it actually is. Token values are read from the browser at
         runtime, so this page reflects the live cascade rather than what the
@@ -110,11 +113,6 @@ const TYPE_SPECIMENS = [
         the system has no shared implementation, it is marked missing rather
         than mocked up.
       </p>
-      <p class="lede">
-        For the other half — the design system as a record you can edit, with
-        family&nbsp;&rarr;&nbsp;app inheritance — see
-        <router-link to="/design-systems">design systems</router-link>.
-      </p>
     </header>
 
     <!-- Drift: what the rulebook claims vs what the tokens do. -->