combo_box: Add ComboBox component

[regexident]

Description

Adds two new components, ComboBox and MultiComboBox, for single- and multi-selection from a searchable list of items, with a free-form text input as the trigger. Both components support custom item rendering via a new item_renderer hook on the underlying searchable list adapter.

To avoid duplicating the substantial dropdown/search/keyboard-navigation logic that already lived inside Select, the searchable-list machinery has been extracted into a new shared searchable_list module which now backs Select, ComboBox, and MultiComboBox:

• searchable_list::adapter::SearchableListAdapter — the shared dropdown/search/keyboard-navigation engine, with a new item_renderer hook so consumers can fully customize how each item is rendered.
• searchable_list::delegate::{SearchableListDelegate, SearchableListItem} — the trait surface that data sources implement.
• searchable_list::item::SearchableListItemEl — the default item element.
• searchable_list::vec::{SearchableVec, SearchableGroup} — flat and grouped in-memory data sources.

Select was rewritten on top of this shared module, dropping ~615 lines of now-duplicated code while preserving its public API.

A ComboBoxStory was added to the gallery with 10 demo sections exercising both components (sizes, disabled state, custom rendering, grouped items, etc.), and full English and Simplified Chinese documentation pages were added at docs/docs/components/combo-box.md and docs/zh-CN/docs/components/combo-box.md (⚠️ obligatory: needs proof-reading ❗ ). The existing select.md pages were updated to cross-link to the new combo-box docs. New i18n strings were added to crates/ui/locales/ui.yml.

Screenshot

Before	After
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Break Changes

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How to Test

1. Run the story gallery: cargo run.
2. Navigate to the Combo Box story in the gallery sidebar and walk through the 10 demo sections, verifying:
   • Typing filters the dropdown items in real time.
   • Keyboard navigation (Up/Down/Enter/Escape) works for both ComboBox and MultiComboBox.
   • MultiComboBox supports multi-selection and shows selected items as removable chips.
   • Custom-rendered items (via item_renderer) display correctly.
   • Grouped items render with their group headers.
   • Disabled and sized variants behave as expected.
3. Navigate to the Select story and confirm there are no visual or behavioral regressions after the rewrite onto searchable_list.
4. Open docs/docs/components/combo-box.md and docs/zh-CN/docs/components/combo-box.md and verify both render correctly and the cross-links from select.md resolve.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

Please use this icon.

[huacnlee]

MultiComboBox is not a good name.

If we add ComboBox, we need do like Shadcn to have multiple option for multiple case.

[regexident]

@huacnlee Both your points should be resolved now. 👍

[huacnlee]

Thank you.