Base form expiration on server time

[OllieinCanada]

Summary

• expose the server Unix timestamp through Nextcloud initial state on internal and public form pages
• derive current server time with performance.now(), keeping expiration checks independent of the client's wall clock even if it changes after page load
• use the server-relative time in every frontend form-expiration decision point
• add a controller unit test that verifies the server timestamp is provided

Rationale

The backend already decides expiration with server time, but the frontend compared form.expires with moment().unix(). A client clock set in the future could therefore hide a form that the server still considers active. Anchoring a monotonic timer to the server timestamp keeps the UI consistent with backend authorization without repeated network requests.

Validation

• Prettier check on all touched frontend files
• ESLint on all touched frontend files
• git diff --check
• PHP unit and clean production-build coverage delegated to CI because PHP is unavailable locally and the Windows npm install had incomplete package extraction

Fixes #3598

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/Controller/PageController.php	50.00%	1 Missing ⚠️

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