Add bounded admin performance monitor

[mehul0810]

Closes #139

Summary

• add an opt-in Admin Performance Monitor toggle and dedicated report
• aggregate admin page, registered AJAX, REST, and cron-adjacent timing, peak memory, and query counts
• show repeated contexts with explicit review thresholds and next-step guidance
• provide an administrator-only, nonce-protected clear action
• retain desktop and 390px report screenshots in hosted WordPress smoke proof

Privacy and overhead boundaries

• disabled by default; disabled mode registers no capture hooks and performs no report writes
• no SAVEQUERIES, query text, full URLs, query arguments, payloads, IPs, usernames, emails, or nonces
• invalid or unregistered AJAX action identifiers become unknown-action
• per-site non-autoloaded storage, 14-day retention, hard cap of 50 aggregate contexts
• no frontend/cache behavior changes or automatic remediation

Validation

• 118 PHP tests / 338 assertions
• 26 JavaScript tests
• PHP lint and scoped PHPCS
• PHPStan
• JavaScript and CSS lint
• production frontend build
• Studio read-only proof: disabled, zero contexts, 14-day/50-context contract, no storage option created
• hosted WordPress desktop/mobile, clear, disabled/enabled, and lower-privilege 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]

Hosted validation is green on head 9966788904a7a61b61d96c380d2b6732bcd754b2.

Visual-proof review:

• The first green smoke artifact revealed that the six-column report clipped important content at 390px.
• Follow-up 9966788 replaced the narrow table with readable per-context cards.
• Fresh run 31456877571 passed and produced desktop/mobile screenshots; both were manually reviewed. Desktop retains the compact table, mobile exposes every metric and guidance without document overflow.

Additional local proof under the required Node 24 runtime:

• JS lint: pass
• CSS lint: pass
• JS unit tests: 26/26 pass
• production frontend build: pass
• git diff --check: pass