Add auto-save to localStorage for Playground editor

[Mossaka]

Summary

• Save editor content to localStorage on every document change so users don't lose work when closing/refreshing the tab
• On page load, restore saved content from localStorage, falling back to the default Hello World template
• Clear the sample dropdown when restoring saved content to avoid a misleading selection
• Wrap localStorage.setItem in try/catch for resilience when storage is full or unavailable

This is a ~10-line change to docs/public/editor/editor.js.

Test plan

• Load the Playground, edit content, close/refresh the tab -- content should be restored
• Select a sample template from the dropdown -- it should replace the editor content and auto-save
• Clear localStorage.removeItem('gh-aw-playground-content') in devtools, reload -- should fall back to Hello World
• Verify theme preference (gh-aw-playground-theme) is unaffected
• Verify deep-link hash URLs still override saved content

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds auto-save functionality to the Playground editor, automatically persisting editor content to localStorage on every document change and restoring it on page load. This prevents users from losing their work when closing or refreshing the browser tab.

Changes:

• Auto-save editor content to localStorage on every document change
• Restore saved content from localStorage on initialization, with fallback to default Hello World template
• Clear sample dropdown when restoring saved content to avoid misleading selection state

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