Board: let a BoardItemTitle inside an item body render in the item header #108

Summary

Let content inside a Board item put its title row in the item header, and tell content whether it is rendered inside a board item.

Why

Board items take a title node and a description (#68). backend.ai-webui renders each dashboard card as one component whose title row (title, help tooltip, header controls, a "View all" link) is built by the card itself, from data the board does not have. To get that row into the board header without restructuring every card, webui currently:

• passes an empty placeholder node as every item's title, wraps the description in its own context, and lets BoardItemTitle inside the body createPortal itself into the placeholder (packages/backend.ai-ui/src/components/BAIBoardItemTitleSlot.tsx, BAIBoard.tsx, BAIBoardItemTitle.tsx);
• hides an empty header and re-pads the content by targeting Board internals: .uic-board-item__header:has(> .uic-board-item__title > …:empty) and + .uic-board-item__content { padding-block-start: var(--board-item-padding-top) } (BAIBoard.css);
• strips the injected title / description back out of onItemsChange so they are not persisted;
• branches in eight card components on "am I inside a board item?" to drop their own inset.

The CSS depends on Board's private DOM and will break silently when it changes.

Proposal

• Export a BoardItemContext / useIsInBoardItem() from Board, true inside an item's description.
• When a BoardItemTitle renders inside an item's description and the item has no title, render it in that item's header (same layout and drag-handle alignment as a title node). An inline prop keeps it in the body when wanted (e.g. a second section title).
• When an item has neither a title nor a body BoardItemTitle, render no header row and keep the board's top padding on the content.

Expected

• webui keeps BAIBoard / BAIBoardItemTitle as thin adapters and deletes the portal slot, the .uic-board-item__* CSS and the onItemsChange stripping.
• Moving or resizing an item never duplicates its title row.

Consumer: lablup/backend.ai-webui#10627 (dashboard card headers).