On latest `dev`, `common/Checkbox.svelte` renders a `<button type="button">` containing only `aria-hidden="true"` SVGs. It has no `role`, no `aria-checked` and no accessible name, and the component has no `$$restProps` spread, so a caller cannot supply a name either.
Assistive technology announces every one of these as an unnamed "button". A screen reader user cannot tell that the control is a checkbox, cannot tell whether it is on or off, and cannot tell what it toggles. The visible label is always an unassociated sibling element, for example `Capabilities.svelte` puts it in a preceding `<div>` with no `id`, and `Groups/Users.svelte` puts it in a different table cell from the checkbox.
Breaks WCAG 4.1.2 Name, Role, Value (Level A) on all three counts at once.
Fix: expose `role="checkbox"` and `aria-checked` on the control, add an `ariaLabel` prop, and pass the label text that is already in scope at each call site. `aria-checked` mirrors the component's existing icon logic exactly, so the indeterminate dash reports `mixed` rather than `false`. The `ariaLabel={ariaLabel || undefined}` shape matches the sibling `common/Switch.svelte`. Every label expression is the same one that renders the visible text next to the checkbox, so the accessible name always matches what is on screen.
Three call sites are deliberately left out of this PR, because they nest `Checkbox` inside another `<button>`, which is invalid HTML and independently broken:
- `workspace/Knowledge/KnowledgeBase.svelte` — the Checkbox's `on:change` sets `includeContent = true` and then the same click bubbles to the outer button, which flips it back with `includeContent = !includeContent`. Clicking the checkbox square is a no-op today, only the text label works. Giving it a confident name would advertise a control that does nothing.
- `workspace/common/MemberSelector.svelte` (two instances) — the inner Checkbox has no `on:change` at all and only works because its click bubbles to the row button. Naming it would create two focusable controls per row with the same name.
Both need the nesting resolved first, so that the row button carries the checkbox semantics. That is a behavioural fix and belongs in its own PR.
Severity: Serious. Affects model capabilities, default features, builtin tools, tool/filter/skill/action selectors and group membership.
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