Make the replacement field optional and neutrally worded on HR-recorded absences

[karlitschek]

"Who covers for you?" is the right question when you are booking your own leave. It is the wrong one when HR records an absence for somebody else — and the requirement behind it was wrong there too: the replacement field was mandatory for any requires_replacement type regardless of who was filling the form in.

The rule belongs to the employee, not to the record

Somebody arranging their own leave knows who can cover and is asked to sort it out before going (§5.1). HR recording or correcting an absence for somebody else is stating a fact, often after the event, and cannot nominate cover on that person's behalf.

So on those paths the field is now offered but never demanded, and reads:

	Label	Placeholder
Your own leave	Replacement *	Who covers for you?
Somebody else's, recorded by HR	Replacement (optional)	Who is the replacement?

The hint follows too — "Optional. A colleague who covers for Lea while they are away, and is notified once this is recorded."

The wording keys off whose leave it is, not off HR mode. So HR recording their own absence — possible since #47 — still gets "Who covers for you?" and the requirement that goes with it, which raw hrMode would have got wrong.

Enforced on both sides, not just hidden: resolveReplacement() takes a required flag, false from create() when onBehalf and from hrEdit(). Without the server half the dialog would allow submit and the API would then reject it.

Also closes a hole

Found while restructuring the check. For a type that did not require a replacement, whatever was submitted was stored unvalidated:

if (!$type->getRequiresReplacement()) {
    return $replacementUid !== null && trim($replacementUid) !== '' ? trim($replacementUid) : null;
}

The guest and self checks sat below that early return, so a guest — or the employee themselves — could be recorded as covering, on sick leave for instance. The method's own docblock claimed "Also rejects guests". Only the demand is conditional; who may be named never was, and now isn't.

Testing

174 tests, 483 assertions, 0 failures (3 new). Each was checked against the original code: the HR-records-for-someone-else case errors without the required flag, and the guest case fails without the restructure. The third pins that self-service still demands a replacement, so the loosening did not leak into the employee path.

eslint 0 errors, stylelint clean, 27/27 vitest, build succeeds. Bundles recompiled in the last commit.

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