ui: track package-wide gaps deferred from the component library PRs

[EhabY]

While documenting known gaps for #1039, an audit of everything shipped in packages/ui so far (#1030, #1031) surfaced package-wide limitations that were never written down or tracked. They are now listed in the package README's "Known gaps" section; this issue tracks fixing them.

• No Button component. The VS Code button style exists only as a descendant selector inside the state panels (StatePanel.css), and the parity story has to fake one. Secondary/danger button colors (--vscode-button-secondary*) have no --ui-* tokens; storybook.css reaches for the raw variables and says so.
• No Loading state panel. Only EmptyState and ErrorState ship; the shared panel skeleton (StatePanel) is internal, so a loading/offline/unauthenticated panel would re-implement it.
• Token layer only covers shipped components. No list/selection-row tokens (needed by the first real list screen), no spacing/typography/z-index/motion tokens (overlay.css hardcodes z-index: 40, StatusPill.css hardcodes font-size: 11px).
• --ui-radius-* tokens only adopted by the overlays. IconButton.css (5px), SearchInput.css (4px), StatePanel.css (4px), and StatusPill.css (11px) hardcode radii, so a future host --vscode-cornerRadius-* change would move overlays but not controls.
• --ui-background assumes sidebar hosting. A webview in an editor tab or panel renders on sideBar-background instead of its host surface.
• useVscodeTheme reports theme kind only. Switching between two themes of the same kind changes no body attribute, so subscribers never re-render; code reading resolved token values in JS goes stale.
• No activity cue under prefers-reduced-motion. The indeterminate ProgressBar renders as a full (complete-looking) bar and the Spinner as a static ring.
• Storybook packages are root-hoisted. src/storybook.tsx and the stories import storybook/test / @storybook/react-vite without declaring them, so stories break on a standalone split the README otherwise designs for.

Prop-level minimalism (controlled-only SearchInput, fixed Spinner/IconButton sizes, no toggle prop) is deliberate for now: the README documents the single-class-specificity CSS override as the escape hatch, and the APIs should grow against real consumers rather than speculatively.

[linear-code]

ENG-3100