Fix successive leave adjustments replacing each other instead of adding up

[karlitschek]

Reported: Lea has 25 days. HR records "+2 wedding" → 27. HR then records "−2, booked in error" → 23, when it should be back to 25.

What was wrong

Not the sum. getEntitlement() is base + carry_over + manual_adjustment, which is right. The value fed into it was wrong.

manual_adjustment is a running total, but the dialog offered it as a per-correction figure labelled "Manual adjustment (+/−)", and update() assigned rather than added:

$ent->setManualAdjustment($adjustment);   // replaces, never accumulates

So the second correction overwrote the first instead of cancelling it:

Step	HR types	Stored adjustment	Allowance
start	—	0	25
"+2 wedding"	2	2	27 ✓
"−2 booked in error"	-2	-2 (replaces 2)	23 ✗

The fix

HR thinks in corrections, so the API now takes them:

• adjustmentDelta adds to the stored total — +2 then −2 cancel to 0, and the allowance returns to 25.
• manualAdjustment still sets it outright, for the rare wholesale overwrite.
• Sending both is refused rather than guessed at.

Applying the delta server-side rather than read-modify-write in the client also means two people adjusting the same entitlement can no longer silently lose one another's correction.

The dialog now asks for the correction and shows the accumulated total beside it, so it is visible that the field adds rather than replaces:

Adjust by (+/−) _____
Adjustments so far: +2 · leaving the field empty changes nothing

Left empty it changes nothing, so saving a base-days edit on its own no longer disturbs the adjustment — which the old absolute field did every time.

Testing

166 tests, 461 assertions, 0 failures (3 new). The regression test walks your exact numbers and, against the old code, fails with precisely the reported result:

Failed asserting that -2.0 is identical to 0.0.

— i.e. 25 + (−2) = 23. eslint 0 errors (no new warnings), stylelint clean, 27/27 vitest, build succeeds. Bundles recompiled in the last commit.

Note on existing data

Rows already saved keep whatever total they hold, and the change history added in #45 starts from here. If Lea currently reads 23, her stored adjustment is −2; one correction of +2 now puts her back to 25 and leaves a recorded reason for it.

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