Add keyboard and screen-reader access to the visualizer

[raphant]

Problem

The canvas has no accessible name or keyboard navigation.
Several icon-only controls do not have accessible labels.
Dynamic agent state has no live-region alternative, and animations do not honor reduced-motion preferences.

Proposed change

Add semantic labels, visible focus, keyboard navigation, a textual graph alternative, and reduced-motion behavior.

Acceptance criteria

• Every icon-only button has an accessible name.
• Keyboard users can select agents, tools, discoveries, and timeline events.
• The canvas has a labeled textual alternative with equivalent essential information.
• Popups and command palettes manage focus and close with Escape.
• Reduced-motion mode disables decorative motion without hiding state changes.
• Automated checks and one manual screen-reader checklist cover the core workflow.

Source

• web/components/agent-visualizer/canvas.tsx owns canvas interaction.
• web/components/agent-visualizer/shared-ui.tsx owns shared close controls and panels.
• web/app/globals.css owns global animation and focus styles.