Add Perform dashboard overview

[mehul0810]

Summary

• Adds a read-only Dashboard tab as the first screen in the existing Perform admin app.
• Surfaces plugin version, docs/support links, runtime diagnostic counts, cache activity, Assets Manager rule coverage, and local release-candidate notes.
• Uses existing local options only; no remote calls, no tracking, no writes, and no historical byte-savings claims.

Validation

• ./vendor/bin/phpcs -s src/Admin/Actions.php src/Admin/Settings/DashboardPayload.php tests/unit-tests/tests-dashboard-payload.php
• composer phpstan
• composer test (45 tests, 99 assertions)
• PATH="/Users/mehulgohil/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin:$PATH" npm run lint:js
• PATH="/Users/mehulgohil/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin:$PATH" npm run lint:css
• PATH="/Users/mehulgohil/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin:$PATH" npm run build
• git diff --check

Proof Gap

• Browser screenshot proof was not captured locally because Docker/wp-env is unavailable in this environment (docker: command not found). The dashboard is implemented through the existing bundled admin app and compiled successfully.

Closes #7

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