feat: add the components Strata's foundation needs

[AmrMsCLL]

What changed

New components for Strata's phase 2 foundation:

• Tabs: route tabs using the existing tab-link style, with aria-current on the active tab.
• TagChip: a coloured chip from a fixed palette (tagColors: gray, red, orange, yellow, green, teal, blue, purple, pink). It can be a link or a plain label, with an optional named remove button.
• Avatar: the initial, in two sizes, with an optional presence dot that is announced to screen readers.
• CodeInput: a one-time-code field (autocomplete="one-time-code", numeric input mode, length and pattern), built on TextField.
• Sidebar: labelled sections of links with icons and capped badges.
• AppShell: a new optional sidebar prop. The sidebar sticks beside the page from md up.
• CommandPalette:
  • a modal combobox with grouped results;
  • arrow keys that wrap across groups, Enter runs the active result, Escape closes it, and focus returns to whatever opened it;
  • a bottom sheet on phones;
  • useCommandShortcut opens it with Ctrl+K or Cmd+K.
• The search icon is added.

Verification

• npm run lint, npm test (7 files, 33 tests) and npm run build pass. The build includes the type-check and the preview.
• New specs cover every component's ARIA and keyboard behaviour, plus AppShell with a sidebar.
• In the local preview, I checked the tabs and tag chips by eye. For the avatars, code input, sidebar and command palette, I checked their computed sizes, colours and active states.

Checklist

• The change is focused and contains no credentials or generated secrets.
• Documentation and tests were updated when behavior changed.
• Deployment or rollback considerations are described when relevant. Nothing deploys; release 0.2.0 follows as its own PR.