Add aria-label guidance and defaults for icon-only controls

[xBrokkr]

Icon-only controls (copy-button, confirm-action triggers, sidebar collapse toggles, pagination arrows, etc.) need accessible names.

Task:

1. Grep the free set for buttons whose only child is an icon.
2. Where the component knows its purpose (e.g. copy-button), add a sensible default aria-label if missing.
3. Where the consumer must supply it, make the prop explicit and document it in the component's prop interface.

Good first issue: self-contained, no design decisions, npm run typecheck guards the prop changes.

[xBrokkr]

Audited every icon-adjacent button across components/core and components/blocks (31 files that import lucide-react). Most already had aria-label, visible text, or an sr-only span (dialog/sheet/drawer close buttons, copy-button, number-stepper, pagination, multi-select, tag-input, password-input all fine).

Three real gaps, fixed in 891eafd:

• calendar: previous/next month buttons were icon-only with no name → added aria-label
• command: the clear-search (X) button had no name → added aria-label="Clear search"
• sidebar: SidebarGroup's collapse toggle had no name at all when the sidebar is collapsed (icon-only in that state), and was missing aria-expanded (required by the disclosure pattern). SidebarItem relied on the title attribute alone as an accessible-name fallback, which screen readers handle inconsistently; it now sets aria-label explicitly too.

All changes are additive attributes, no behavior or markup changes; npm run typecheck still passes clean.