Adds @hive/shared/tabs.js — createTabStrip(tabbar, {defaultId, onShow}):
a hash-routed tab strip that resolves controls by `[data-tab="<id>"]`
inside the passed container and panels by `[data-tab-pane="<id>"]`,
toggling `.hive-tab--active` + aria-selected + the panel's `hidden`, and
firing onShow(id) for per-tab side-effects. Always hash-routed (deep-
linkable + back/forward), no element-resolver callbacks (convention over
config), per the #1464 design review.
Wires it up: @hive/shared exports `./tabs.js` + `./tabs.css`, and
common.css @imports tabs.css (component structure, not a swap target, so
inlining is fine — unlike theme.css).
Migrates the logs sub-tabs as the first consumer: markup uses
`.hive-tab`/`data-tab`/`data-tab-pane`, logs.css drops the duplicated
base styles (keeps only its `flex:1` layout delta), and logs.js swaps its
activeTab/showTab/hashchange for createTabStrip (onShow lazy-loads the
SYSTEM tab). Behaviour-preserving. aria-selected is now standardised.
First piece of the generic tabs component: @hive/shared/tabs.css carries
the base .hive-tabbar / .hive-tab / .hive-tab--active styles (lifted from
the logs sub-tab pattern, the most generic of the three tab UIs). Colours
use the semantic theme vars so it inherits theme-swap safety.
Not yet consumed — the createTabStrip JS helper + the per-call-site
migrations (logs, agent window-tabs, dashboard tabbar) follow. Count
pills + the dashboard's responsive overflow menu will layer on top in
the per-page CSS rather than the shared base.
