[personal-wp] Introduce a user-focused Personal Playground

[akirk]

This is a tracking issue for creating a Personal Playground which will be a deployment on a new URL that removes some features of playground.wordpress.net and adds some others. This is to create a version that is more enduser-focussed.

Removes:

• Blueprint gallery / site list (replaced with a tailored overlay)
• Import from file/URL/GitHub UI
• Create new temporary site option
• Multiple saved sites management

Adds:

• OPFS storage by default (persistent site)
• Welcome plugin with onboarding
• Automatic language detection from browser
• App catalog (pre-configured blueprints to install)
• Backup reminder system with history
• Health Check integration for crash recovery
• Multi-tab coordination (one worker per site, dependent tabs, takeover protocol)
• Cross-tab sync for backup status

PRs:

• [website] Refactor out an Overlay component from saved-playgrounds-overlay #3152
• [personal-wp] Introduce a Personal Playground mode #3155
• [personal-wp] Add Health Check recovery #3157
• [personal-wp] Backup reminder UI #3162
• [personal-wp] Add multi-tab coordination #3163
• [personal-wp] Add app catalog #3173
• [Personal-WP] Add GitHub OAuth integration for private repositories #3180
• [Personal-WP] Improve recovery mode and add direct OPFS file browser access #3182
• [personal-wp] Add deployment workflow for my.wordpress.net #3184
• Blueprints: Blueprint and Apps for your Personal Playground #166