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/credentials.js

@@ -21,7 +21,7 @@
 
 import { $, esc, fmtAgeSecs, renderServerWarnings } from './common.js';
 import { el } from '@hive/shared/dom.js';
-import { createTabStrip } from '@hive/shared/tabs.js';
+import '@hive/shared/hive-tab-strip.js';
 import { themedConfirm, themedToast } from '@hive/shared/modal.js';
 
 let agents = [];
@@ -507,7 +507,12 @@ async function init() {
   $('gh-form').addEventListener('submit', submitGithub);
   $('ef-form').addEventListener('submit', submitForgeAccount);
 
-  createTabStrip(document.getElementById('cred-tabbar'), {
+  document.getElementById('cred-tabbar').configure({
+    tabs: [
+      { id: 'matrix', label: 'MATRIX' },
+      { id: 'github', label: 'GITHUB' },
+      { id: 'forges', label: 'FORGES' },
+    ],
     defaultId: 'matrix',
   });