command: Add a Command palette component (#2765)

Adds a shadcn-style `Command` palette built on GPUI's virtual list, with
local search, groups, disabled and checked items, Action keybinding
hints, custom rows, different row heights, loading/empty states,
headers, footers, and keyboard navigation.

`Command` owns the render-time model and callbacks; `CommandState` keeps
only interaction state such as query, selection, focus, loading, and
scrolling. `CommandItem` is treated as a render element rather than a
business-data container. Applications retain their own model and resolve
`on_select` / `on_confirm` through `IndexPath`.

`IndexPath` uses stable input-model coordinates. Items supplied through
`.items(...)` occupy section 0 and retain their original input position
in `row`, even after local filtering. Explicit groups use group/item
coordinates. This matches `ListState` conventions and avoids mapping a
filtered selection to the wrong business object.

The Story app demonstrates grouped commands, custom rows, asynchronous
stock search, Theme preview/confirmation with Escape rollback, headers,
footers, Kbd hints, empty content, and a component palette opened with
`ctrl-shift-p`. The selected border radius is also persisted across
restarts.

## Real-world validation in the Longbridge Pro app

The API was integrated into three production command flows in the
Longbridge Pro app in
[longbridge-gpui#3564](https://github.com/longbridge/longbridge-gpui/pull/3564):

- application Command Palette: custom ranking, localized search terms,
GPUI Actions and keybinding hints, plus close-before-dispatch behavior
- Theme Picker: initial selection, keyboard preview, mouse/keyboard
confirmation, Escape rollback, and protection from deferred preview
callbacks after close
- stock search: asynchronous owner-managed results, loading state,
complex rows with different heights, history/filtering, modal and
embedded use, new-tab confirmation, and AI-focus selection clear/restore

This integration exposed two API design gaps that were fixed here:

- owners can explicitly set or clear selection with
`CommandState::set_selected_index`
- ungrouped `.items(...)` callbacks now return the intuitive original
item position in `IndexPath.row`, rather than encoding each item as a
section

Verification:

- gpui-component Command tests: 32 passed
- `cargo check -p gpui-component-story`
- longbridge-gpui: `cargo check -p action_bar -p command_palette -p
counter_search`
- the Longbridge Pro app was launched and all three migrated production
flows were manually exercised

Docs: `website/docs/components/command.md` and
`website/zh-CN/docs/components/command.md`.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>