frontend: move shared/tabs.js+css into one-dir tabs/

Pure structural move, no API or behaviour change: tabs.js/tabs.css
move into shared/src/tabs/, matching the one-dir-per-component
layout the other shared components already use (hive-btn,
hive-dialog, hive-toast, hive-menu, side-panel).

Both files are consumed exclusively through @hive/shared's
package.json exports map (./tabs.js, ./tabs.css), never by a raw
relative path, so updating the two export targets is the only
change needed -- none of the 9 call sites (dashboard tabbar, logs,
core, builds, credentials, stats x2, agent stats) touch anything.
Verified the built dashboard/agent bundles still resolve both
files correctly.

frontend/packages/shared/src/tabs/tabs.css

@@ -0,0 +1,37 @@
+/* Shared tab-strip chrome — the generic tab/sub-tab styling used across
+   the dashboard tabbar, the logs sub-tabs, and the agent stat-window
+   selector. Behaviour lives in `@hive/shared/tabs.js` (createTabStrip).
+
+   Colours come from the semantic theme layer (theme.css → colors.css),
+   so this inherits theme-swap safety for free. Call sites keep a thin
+   variant class for size/spacing deltas; richer affordances (count
+   pills, the dashboard's responsive overflow ⋮ menu) layer on top in the
+   per-page CSS. See docs/web-ui/css-vars.md. */
+
+.hive-tabbar {
+  display: flex;
+  gap: 0.2em;
+}
+
+.hive-tab {
+  display: inline-flex;
+  align-items: center;
+  padding: 0.35em 0.9em;
+  border-radius: 4px;
+  color: var(--subtext0);
+  text-decoration: none;
+  font-size: 0.85em;
+  letter-spacing: 0.05em;
+  cursor: pointer;
+  transition: background 100ms, color 100ms;
+}
+
+.hive-tab:hover {
+  background: var(--border);
+  color: var(--fg);
+}
+
+.hive-tab--active {
+  background: var(--border);
+  color: var(--purple);
+}

frontend/packages/shared/src/tabs/tabs.js

@@ -0,0 +1,71 @@
+// Shared hash-routed tab strip — the generic behaviour behind the
+// dashboard tabbar, the logs sub-tabs, and the agent stat-window
+// selector. Pairs with `@hive/shared/tabs.css` (the .hive-tab* styles).
+//
+// Convention over configuration (no element-resolver callbacks):
+//   - Tab controls are the `[data-tab="<id>"]` elements inside the
+//     `tabbar` you pass in (any element — `<a>` or `<button>`; give
+//     them class `hive-tab`).
+//   - Panels are matched by `[data-tab-pane="<id>"]` anywhere in the
+//     document.
+//
+// Routing is always via `location.hash` so tabs are deep-linkable and
+// honour back/forward. A page may only host one strip (one hash); if a
+// second independent strip is ever needed we add a key prefix then.
+//
+// On show: toggles `.hive-tab--active` + `aria-selected` on each control
+// and `hidden` on each panel, then fires `onShow(id)` for per-tab
+// side-effects (lazy loads, etc.). See docs/web-ui/css-vars.md.
+
+/**
+ * @param {Element|null} tabbar  container holding the `[data-tab]` controls
+ * @param {{ defaultId?: string, onShow?: (id: string) => void }} [opts]
+ * @returns {{ show: (id: string) => void, active: () => string|null }}
+ */
+export function createTabStrip(tabbar, opts = {}) {
+  const { defaultId, onShow } = opts;
+  if (!tabbar) {
+    return { show() {}, active: () => null };
+  }
+  const tabs = Array.from(tabbar.querySelectorAll('[data-tab]'));
+  const ids = tabs.map((t) => t.getAttribute('data-tab'));
+  const fallback = defaultId && ids.includes(defaultId) ? defaultId : ids[0];
+
+  const paneFor = (id) => document.querySelector(`[data-tab-pane="${id}"]`);
+
+  const active = () => {
+    const hash = location.hash.replace(/^#/, '');
+    return ids.includes(hash) ? hash : fallback;
+  };
+
+  const show = (id) => {
+    const target = ids.includes(id) ? id : fallback;
+    for (const tab of tabs) {
+      const tid = tab.getAttribute('data-tab');
+      const on = tid === target;
+      tab.classList.toggle('hive-tab--active', on);
+      tab.setAttribute('aria-selected', String(on));
+      const pane = paneFor(tid);
+      if (pane) pane.hidden = !on;
+    }
+    if (typeof onShow === 'function') onShow(target);
+  };
+
+  // Drive every activation through the hash so deep-links + back/forward
+  // stay authoritative; the hashchange listener does the actual showing.
+  for (const tab of tabs) {
+    tab.addEventListener('click', (e) => {
+      e.preventDefault();
+      const id = tab.getAttribute('data-tab');
+      if (location.hash.replace(/^#/, '') === id) {
+        show(id); // same hash → no hashchange, re-show directly
+      } else {
+        location.hash = id;
+      }
+    });
+  }
+  window.addEventListener('hashchange', () => show(active()));
+
+  show(active()); // initial render from the current hash
+  return { show, active };
+}