docs: expire stale ad click ids before sending conversions to the worker

[pubkey]

This PR contains:

A BUGFIX (docs site conversion tracking — docs-src).

Describe the problem you have without this PR

storeAdClickId() (docs-src/src/theme/Root.tsx) saves the gclid/gbraid/wbraid from the landing URL into localStorage under click_id, and getStoredAdClickId() (docs-src/src/components/trigger-event.tsx) reads it back for every tracking event to attribute conversions sent to the offline-conversion worker.

The stored id never expired. So once a visitor arrives from an ad, getStoredAdClickId() keeps returning that same click id indefinitely — on every later visit, including organic return visits weeks or months later. Every one of those events is then posted to /api/google-ads attributed to the old click. Two concrete problems:

• Google Ads rejects the upload with "This click is too old" once the click is past the conversion window (observed on the live offline-conversion upload).
• Even inside the window, crediting a much-later visit to a long-past ad click is mis-attribution — the worker keeps uploading conversions against a click the visit had nothing to do with.

The fix

Ignore — and clear — a stored click id once it is older than 90 days (Google Ads' maximum click-through conversion window). After that a click can never be legitimately imported anyway, so getStoredAdClickId() returns null and removes the stale entry. The client then only ever calls the worker with a click id that can still be attributed. Also hardens against a missing/invalid timestamp.

Behaviour is unchanged for normal (fresh) ad clicks; only stale ids are dropped.

Todos

• Tests — the change lives in docs-src (Docusaurus site tracking code), which has no unit-test harness in this repo (the test/ suite covers the library, not the docs site). The fix is a small, self-contained localStorage-age guard; happy to add a test if you point me at where docs-site logic should be tested.
• Documentation — inline comment explains the 90-day window and why.
• Typings — unchanged (same return type).
• Changelog — N/A (docs-site tracking, not the published package).

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