refactor(frontend): add createTabStrip + migrate logs sub-tabs (#1464 step 1)

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.

frontend/packages/shared/package.json

@@ -8,6 +8,8 @@
   "exports": {
     ".": "./src/index.js",
     "./terminal.js": "./src/terminal.js",
+    "./tabs.js": "./src/tabs.js",
+    "./tabs.css": "./src/tabs.css",
     "./colors.css": "./src/colors.css",
     "./theme.css": "./src/theme.css",
     "./base.css": "./src/base.css",