fix(ui): close package-wide gaps from the component library PRs

[EhabY]

🤖 This PR was created by Coder Agents on behalf of @EhabY.

Closes #1046 (all 8 checkboxes) plus the overlays gaps deferred from #1039; list/selection-row tokens stay deferred to the Tree suite (#1037). One squashed commit, after a review-and-simplify pass.

Changes

• packages/ui declares its Storybook devDependencies; the root storybook pin moves into the pnpm catalog.
• Tokens: secondary-button colors, --ui-font-size-small, --ui-z-index-overlay, --ui-panel-background for non-sidebar hosts, and VS Code's cornerRadius and spacing scales (spacing mirrored in full; names are px times ten). Controls adopt the radius scale and state panels the spacing scale; metrics VS Code hardcodes natively (button/hover/menu paddings, the 5px hover radius) stay hardcoded.
• Button matching monaco-text-button, with a secondary variant; the state panels and story triggers use it instead of faking one.
• LoadingState panel reusing the StatePanel skeleton stylesheet, with the large Spinner in the icon slot.
• Menus gain *CheckboxItem, *RadioGroup/*RadioItem, *Label, and *Keybinding, styled like native. Keybinding hints take a contribution's key/mac/win/linux fields and render the current OS's binding in the native label style (⇧⌘R on macOS, Ctrl+Shift+R elsewhere); formatKeybinding is exported for other surfaces. VS Code has no API to resolve user-remapped bindings, so hints show contributed defaults (documented known gap).
• useVscodeTheme re-renders on same-kind theme switches: one shared observer, snapshot of theme id + kind.
• Reduced motion keeps activity cues: striped indeterminate ProgressBar band, Spinner arc over a tinted ring.
• Button parity fixes surfaced by the vscode-elements comparison: focus outlines apply on :focus like native (a click shows the ring; workbench style.css outlines button:focus), a toggled IconButton keeps its active background on hover (native applies checked colors as inline styles in toggle.ts), and IconButton no longer defaults a browser title box (native hints via the styled hover widget; wrap in Tooltip).
• TooltipProvider becomes a public export mounted once per app, so hovers moving between triggers skip the 500ms delay like native; tooltips cap at half the window height. Tooltip has no per-instance delay prop; nest a provider where a different delay is needed.

Testing

pnpm typecheck, lint, format:check, test:webview (315 passed), and storybook:ci pass. Button/control geometry and secondary colors pixel-verified against the captured VS Code themes; the menu, tooltip, and loading stories driven in a real browser. The parity story also compares the secondary Button and an open menu (items, separator, keybinding hint) against vscode-elements. Pixel CI will show intended diffs where stories now use Button as the overlay trigger.