feat: Add a new local A2UI JSON viewer in the shell app.

[dmandar]

Description

Also create sample A2UI JSON files to show up.
Can be opened with the ?/app=local query param after running shell app. e.g. http://localhost:5173/?app=local

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Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

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[gemini-code-assist]

Code Review

This pull request introduces a local offline previewer and prototyping tool, adding a file upload interface, built-in JSON samples, and a glassmorphic toast notification system. The A2UILayoutEditor component was updated with state management for local mode, enhanced styling, and logic to process local actions. Review feedback focuses on improving code quality and robustness, specifically by removing redundant whitespace, strengthening type safety for the markdown renderer, restoring encapsulation for the snackbar accessor, and refining the toast notification logic to prevent race conditions.