frontend: add <hive-tab-strip>, convert logs/credentials/core/builds tabbars

Every sub-page tabbar (logs.html, credentials.html, core.html,
builds.html) hand-wrote the same <nav class="hive-tabbar"><a
class="hive-tab">...</a></nav> boilerplate and then called
createTabStrip() on it after the fact. Add <hive-tab-strip>, a
markup-owning custom element (same reuse-boundary pattern as
<hive-menu>/<hive-side-panel>) that renders that markup from a
declarative tabs list, then wires the existing createTabStrip()
behaviour over what it just rendered — no behaviour duplication.

Convert all four sites to use it: each page's JS now calls
`.configure({ tabs, defaultId, onShow })` on the tabbar element instead
of `createTabStrip(el, opts)`, and configure() returns the identical
{ show, active } shape so nothing downstream changes. builds.js's
rebuild-queue count pill (builds-tab-count-rebuild) is expressed as a
tab's `badgeId` and renders nested in the same spot.

The dashboard's own tabbar and the two no-pane stats time-range
pickers are a different markup/behaviour shape and are intentionally
left alone.

frontend/packages/dashboard/src/logs.html

@@ -19,24 +19,8 @@
        moved to /builds.html (the build lifecycle hub). -->
   <header class="page-header">
     <a class="page-back" href="/">← home</a>
-    <nav class="hive-tabbar logs-tabbar" id="logs-tabbar" role="tablist">
-      <a class="hive-tab" id="logs-tab-agent" href="#agent" role="tab"
-         aria-controls="logs-pane-agent" data-tab="agent">
-        <span class="logs-tab-label">AGENT</span>
-      </a>
-      <a class="hive-tab" id="logs-tab-infra" href="#infra" role="tab"
-         aria-controls="logs-pane-infra" data-tab="infra">
-        <span class="logs-tab-label">INFRA</span>
-      </a>
-      <a class="hive-tab" id="logs-tab-system" href="#system" role="tab"
-         aria-controls="logs-pane-system" data-tab="system">
-        <span class="logs-tab-label">SYSTEM</span>
-      </a>
-      <a class="hive-tab" id="logs-tab-audit" href="#audit" role="tab"
-         aria-controls="logs-pane-audit" data-tab="audit">
-        <span class="logs-tab-label">AUDIT</span>
-      </a>
-    </nav>
+    <hive-tab-strip class="hive-tabbar logs-tabbar" id="logs-tabbar" prefix="logs"
+                    role="tablist"></hive-tab-strip>
   </header>
 
   <main class="logs-main">