fix(ui): align announcement close controls and restore voice call backdrop blur

[kev1n77]

Summary

• Replace the announcement toast’s custom close button with the shared IconButton, keeping the countdown ring and a transparent background.
• Remove entrance scaling while preserving the toast’s fade and horizontal slide.
• Add the Figma-specified 10px backdrop blur to the voice call panel.

Type and Areas

Type: Bug fix / UI/UX

Areas: Web UI, shared design-system components

Motivation / Impact

The announcement close icon was oversized, and its presentation did not match the component library. The updated control uses the shared small glyph and aligns its hit area with the countdown ring. Removing entrance scaling also avoids resampling the icon during the opening animation.

The voice call panel previously showed sharp application content through its translucent background. It now combines an 80% dark fill with backdrop blur.

Verification

• pnpm run check:web — passed.
• Design-system package builds passed; pnpm --dir design-system run build:lab passed on retry after overlapping builds temporarily removed generated declarations.
• pnpm --dir src/web-ui run test:run src/shared/announcement-system/store/announcementStore.test.ts — 4 tests passed.
• node --test design-system/packages/ui/tests/voice-call.test.mjs design-system/packages/ui/tests/icon-button.test.mjs — 11 tests passed.
• pnpm run motion:audit — completed; no duplicate global keyframe names.
• git diff --check — passed.

Reviewer Notes

• Automatic dismissal and countdown timing remain unchanged.
• Browsers without backdrop-filter support retain an opaque voice call background. Reduced-transparency and forced-colors modes disable blur.
• No persistence, protocol, or user-facing string changes.
• Final animation fidelity still requires manual verification.
• Verification was local; remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.