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/shared/package.json

@@ -10,6 +10,7 @@
     "./terminal.js": "./src/terminal/terminal.js",
     "./tabs.js": "./src/tabs/tabs.js",
     "./tabs.css": "./src/tabs/tabs.css",
+    "./hive-tab-strip.js": "./src/tabs/hive-tab-strip.js",
     "./colors.css": "./src/colors.css",
     "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",