feat(ui): add Radix overlays themed to VS Code

[EhabY]

Closes #1038 (DEVEX-619)

Stacked on #1030 (the packages/ui scaffold); this diff is overlay-only.

What

• Tooltip, ContextMenu, DropdownMenu wrapping @radix-ui/react-* — the package's only runtime deps besides codicons; React stays a peer.
• Menus expose Radix's compound parts as flat named exports; Tooltip is a single component with a content prop (Radix Themes / Fluent v9 shape) and a 500ms delay matching workbench.hover.delay.
• Styled to the native widgets via new --ui-menu-* / --ui-tooltip-* tokens in tokens.css (menu colors from menu.* IDs, tooltip from editorHoverWidget.*, shadow from widget.shadow which self-removes in HC). Metrics verified against the VS Code source: 24px items, 2em check-column gutters, corner-radius tokens with the pre-token 5px/4px fallbacks.
• Inherit-first: overlay CSS declares no typography — portalled content inherits from body, matching real webview default styles (Storybook's global.css stand-in moved typography from #root to body accordingly).
• High contrast via the VS Code contrast variables, plus forced-colors (system colors re-pointed through custom properties) and prefers-reduced-motion handling.
• Long content caps to the available height Radix reports and scrolls, like native menus/hovers.
• Library boundary: ESLint now rejects @repo/* and relative cross-package imports in packages/ui; shared internals go through #cx/#storybook subpath imports; sideEffects declared for tree-shaking.

Storybook

One .stories.tsx per overlay. The Open matrix stories (nested submenu, disabled items, separators, icons) snapshot Light, Dark, High Contrast, and High Contrast Light via Chromatic modes; play functions open the overlays pre-snapshot. ManyItems/OverflowScrolls assert the scroll caps.

Webview focus handling

Verified via play assertions (the preview iframe owns its document, like a webview): focus moves into the menu on open, Escape closes and returns focus to the trigger, tooltip opens on keyboard focus.

Manual checklist for the first real webview consumer:

• Escape returns focus to the trigger, not the workbench
• Clicking workbench chrome (webview blur) closes the menu
• Tab from an open menu strands no focus on body
• Check a real HC theme and Windows forced colors

Out of scope

Checkbox/radio items, labels, keybinding hints, a data-driven items wrapper, and Tasks' migration off @vscode-elements.

🤖 Generated with Claude Code

[linear-code]

DEVEX-619