fix: clear stale popovers on bfcache restore

[Shadorux]

Closes #2537

Summary

• Remove stale popover nodes when Safari restores a page from the back/forward cache.
• Reset the active anchor on persisted pageshow so the restored page starts from a clean popover state.
• Add regression coverage for persisted versus ordinary page-show behavior.

This keeps normal loads untouched and only performs the cleanup on a bfcache restore, matching the iOS/Safari reproduction where a synthetic hover popover survives the full navigation to a PDF and reappears blank on Back.

Validation

• node --test quartz/components/scripts/popover.test.ts: 8/8 passed
• npm test: 165/165 passed
• npm run check: TypeScript + repo-wide Prettier passed
• npx quartz build: passed, 56 files emitted
• git diff --check: passed

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