sidebar: Make every menu item a tab stop with a focus ring

[huacnlee]

Problem

SidebarMenuItem was a plain div + on_click; nothing in crates/component/src/sidebar/ called track_focus. GPUI's tab-stop table only lists nodes that painted with a tracked focus handle, so no sidebar item could be reached with Tab, and a keyboard user had no way to open a page.

Reported from a downstream application (ai-desktop/pilot, issue #230 there): the settings dialog's nine pages were unreachable, and the app rebuilt the item on gpui_kit::base::Button for that one reason.

Change

• The row owns a keyed focus handle ((id, "focus")), the same pattern as Button. It is a tab stop by default and activates on Enter / Space through GPUI's keyboard click, so on_click receives ClickEvent::Keyboard.
• Draws focus_ring_style while focused. A left mouse press calls prevent_default, as Button does, so the ring shows only during keyboard navigation.
• Disabled items do not track focus and are skipped.
• New builders: tab_stop(bool), tab_index(isize), and impl FocusableExt (focus_ring(false)) for containers that clip the ring and draw their own.
• Exposes Role::Button and an aria_label from the item label.
• Drops the row-level overflow_x_hidden: it would clip the outward ring, and the label containers inside already clip.

Tests

crates/component/src/sidebar/menu.rs:

• tab_reaches_each_enabled_item_and_enter_activates_it — Tab walks Inbox → Sent → wraps, skipping the disabled item and the tab_stop(false) item; Enter fires each on_click.
• space_activates_the_focused_item.

Both fail on main (no item is ever focused) and pass here.

cargo test -p gpui-component --lib sidebar      # 20 passed
cargo clippy -p gpui-component --all-targets -- --deny warnings
cargo fmt --check

Docs

website/component/sidebar.md and website/zh-CN/component/sidebar.md gain a "Keyboard Access / 键盘可达" section.

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