Add evidence-bound Performance Health Dashboard

[mehul0810]

Closes #178

Summary

• combine existing local site inventory, database audit, runtime diagnostics, cache activity, asset rules, and bounded server signals into one health dashboard
• show Ready, Review, Observing, and Not measured states with plain-language next actions
• keep Core Web Vitals and response-compression claims explicitly separate from local-only evidence
• show store-safety guidance only when a store pattern is measured
• retain desktop and 390px mobile screenshots from the hosted WordPress smoke run

Safety

• no automatic optimization or remediation
• no external requests without a user clicking documentation or support
• no synthetic performance score or estimated savings
• existing capability gate continues to protect the settings page

Validation

• 111 PHP tests / 311 assertions
• 22 JavaScript tests
• PHP lint, scoped PHPCS, PHPStan
• JavaScript and CSS lint
• production frontend build
• Studio runtime payload check for server signals and six stable diagnostic IDs
• hosted WordPress desktop/mobile smoke and screenshot proof required before merge

[github-actions]

WordPress Playground Preview

The changes in this pull request can previewed and tested using a WordPress Playground instance.

[mehul0810]

Final proof: all hosted checks are green. Playwright run 31455649156 passed the real WordPress smoke suite and retained perform-settings-ui-proof with desktop and 390px dashboard screenshots plus no-horizontal-overflow assertion. Manual review confirms clear hierarchy, aligned cards/actions, readable responsive stacking, explicit Not measured states, and no synthetic score or unverified Core Web Vitals claim.