feat(nav): shared sub-tabs component for settings and kitchen

Extracts the tab-bar UI into a reusable renderSubTabs() utility
(public/utils/sub-tabs.js + public/styles/sub-tabs.css) so all
sub-module navigation shares one implementation and visual style.

- Settings: replaces template-literal <nav class="settings-tabs">
  with renderSubTabs(); tabs now show icon + label (pill-style),
  group separators via separatorBefore, panel switching via onChange
- Kitchen: renderKitchenTabsBar() becomes a thin wrapper around
  renderSubTabs(); kitchen-tabs.css keeps only layout adjustment rules
- CSS: kitchen-tab* rules removed from kitchen-tabs.css,
  settings-tab-btn* rules removed from settings.css; both now
  inherit from sub-tabs.css
- tokens.css: adds --sub-tabs-height (52px default;
  kitchen overrides to --kitchen-tabs-height: 56px)
- test-browser-loader.mjs: resolves browser-absolute /utils/*.js
  imports to public/ directory for Node test compatibility

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/index.html

@@ -42,6 +42,7 @@
   <link rel="stylesheet" href="/styles/pwa.css" />
   <link rel="stylesheet" href="/styles/layout.css" />
   <link rel="stylesheet" href="/styles/glass.css" />
+  <link rel="stylesheet" href="/styles/sub-tabs.css" />
   <link rel="stylesheet" href="/styles/kitchen-tabs.css" />
   <link rel="stylesheet" href="/styles/login.css" />