feat: add remaining portable A2UI catalog components

[brianegan]

Summary

Adds the five portable A2UI v0.9 components missing from the Jaspr catalog: Image, Tabs, Modal, AudioPlayer, and Video. Icon remains application-defined because Jaspr has no global icon set.

Changes

• Render native image, audio, and video elements with dynamic A2UI properties.
• Add keyboard-accessible tabs and native dialog behavior without a modal action interceptor.
• Add shared VM and browser fixtures plus exact stylesheet coverage for emitted classes and the dialog backdrop.

Testing

Run ./tool/coverage.sh, cd example && ./tool/coverage.sh, and dart test -p chrome test/browser. Together they cover schemas, rendered markup, live A2UI updates, keyboard focus, and native dialog behavior.

Gates

Gated 4789673 against origin/main@a273745.

• review: skill code-review, pinned fable unreachable, ran on the session model, passed, agent-reported
• test: command, passed in 53.0s
• example-test: command, passed in 14.9s
• browser-test: command, passed in 6.2s

Reviewed against the plan recorded on the issue.

Closes #16