refactor(ui): move the UI components from Radix to Base UI

[EhabY]

Replaces the five @radix-ui/* packages in packages/ui with @base-ui/react. Styling and behavior stay as they were. Where Base UI differs, this PR corrects it or makes the API stricter, as listed in the following sections.

Why now

Nothing outside packages/ui imports it yet, and the menus and Select appear only in stories and tests, so the swap won't get cheaper than this.

What the switch buys

• Five dependencies become one.
• Base UI's context menu reuses the menu's parts, so ContextMenu and DropdownMenu keep only their root, trigger, and surface, and share every item as Menu*. The menuPart factory that bridged the two Radix scopes is gone.
• Every positioner names --available-height, --anchor-width, and --transform-origin the same way, so the per-surface CSS mappings collapse into one overlay rule.
• Radix-only workarounds are deleted: TooltipScope (a Base UI tooltip works without a provider), the jsdom pointer-capture and scrollIntoView stubs, the Select scrollbar overrides, and the tree's hidden hover anchor <span>. The tree's shared hover bubble now positions against a virtual anchor measured from the tree.

API changes

• MenuContent, DropdownMenuContent, ContextMenuContent: these take side, align, sideOffset, and alignOffset for placement, and forward every other prop, including ref, to the menu element.
• SelectContent: className and style dress the panel, and every other prop goes to the listbox.
• Select: requires items and takes a single value. Base UI builds the trigger label from items, so without it the trigger shows the raw value. The type now catches that, and SelectProps is exported.
• MenuLabel: builds on Menu.GroupLabel and names the new MenuGroup or the MenuRadioGroup it sits in. Outside a group, Base UI throws.
• TooltipProvider: delayDuration is now delay, matching the underlying prop.
• onValueChange and onOpenChange: these gain a second eventDetails argument.
• Hover delegate: HoverDelegateScope, HoverDelegate, and HoverTarget are no longer exported. Only Tree uses them, so the public tooltip API is Tooltip and TooltipProvider.

Corrections on top of Base UI

• Every overlay uses a fixed positioner. A fixed positioner gets its own layer, which keeps text antialiasing greyscale instead of fringing into subpixel color.
• The select list and context menu align to the start of their anchor, as they did with Radix. The context menu sets side explicitly, which also skips Base UI's context-menu offsets.
• The select list scrolls a viewport inside the padded panel, so the last row sits against the padding instead of under the corner radius.
• Base UI sets no role="tooltip" or aria-describedby on tooltips, and names a select option from its whole contents. Tooltips get both attributes back, and an option with a description is labelled by its text alone.

Performance

Tree rows still hand their hovers to one shared bubble instead of mounting a tooltip each. In a jsdom benchmark with 2,000 rows (mount plus one rerender), sharing costs about 40 ms with either library, a tooltip per row costs about 300 ms with Radix and about 225 ms with Base UI, and Base UI's detached triggers cost about 220 ms. The React Compiler memoizes the delegate, so moving the hover re-renders no rows.

Verification

• pnpm typecheck, pnpm lint, pnpm test (2756 passed), pnpm build, and pnpm storybook:build pass.
• In Chrome, every overlay, item, and label in the menu, select, and VS Code parity stories lands on the same pixel before and after the API changes, and the tree hover lands where it did with the anchor <span>.
• test/webview/ui/overlays.test.tsx checks that each surface forwards its props to the right element and carries the classes, data-open, and positioner variables the stylesheets read.

Stories drive a component into the state Pixel photographs, so their play functions only interact and wait. The checks nothing else covered moved to unit tests.