Fix/a11y share modal sr focus announcements

[Ovgodd]

Purpose

Improve ShareModal accessibility for screen readers and keyboard users:

• reduce noisy announcements from decorative UI
• add clear, contextual labels on action controls
• keep focus predictable after invite/member actions
• announce add/remove/role-change feedback even when focus stays on the same control

What stays on shared primitives: menu popup semantics and dialog/menu announcement noise are still partly driven by UI kit / Cunningham components. Related work is tracked in ui-kit PR #210 and Cunningham PR #390.

Three commits on purpose (flow + labels, then icon noise cleanup).

Proposal

• Add contextual aria-labels on role, remove-user, and show-more controls (members vs invitations).
• Return focus to the search input after adding or removing a pending invited user.
• Add a polite live region for add/remove user and role-change confirmations.
• Use a short clear-then-set delay so confirmations are still announced when dropdown close noise happens.
• Expose ShareModal section titles as headings (h3) for SR heading navigation.
• Hide decorative Material Icons (search, add, close, arrow_*, link) with aria-hidden="true".
• Verify role/remove/show-more labels, invite focus flow, live confirmations, icon noise cleanup, and heading navigation.
• Document remaining dropdown-close SR dump after role/reach selection as out of scope for ShareModal; expected to improve with ui-kit PR #210 (role="menu" + trigger ARIA instead of dialog popup semantics).