Schema explorer table menu button has no accessible name

## What

The table row menu button in the schema explorer has no accessible name. Its only content is an icon, and it carries neither `aria-label` nor `title`, so a screen reader announces an unnamed button.

## Where

src/components/schema-explorer/TableItem.tsx:273

```tsx

e.stopPropagation()}
>

```

The expand button above it, at line 238, is fine: it carries `aria-expanded` and its content includes the table name as text.

## Expected

The menu button announces what it opens and which table it belongs to.

## How to fix

Add an `aria-label` naming the action and the table, for example `aria-label={`Actions for ${table.name}`}`. A `title` with the same text also gives sighted users a tooltip on a control that is otherwise invisible until hover.

## How to check

Inspect the button in the accessibility tree and confirm it has a name.