Localize editor: fast double-commit races the annotation refetch and POSTs a duplicate

Found while reviewing #386, but the hazard predates it.

Mechanism

modalContext.existingAnnotation on LocalizeAlertPage is derived from the react-query cache, and saveDetection.onSuccess only invalidates — the value stays null until the refetch lands. saveDetectionReview branches create-vs-patch on it, so two commits in quick succession on a frame with no annotation yet both take the POST branch. The second POST hits uq_detection_annotation_detection_id and the annotator sees a "Failed to save frame — try again" toast for a save that actually succeeded.

Nothing debounces or disables between the two writes: useMutation.mutate has no in-flight guard and isSaving only feeds the "Saving…" label.

Triggers

Any two fast commits on an undecided frame: double-press of Delete/Backspace (auto-repeat is already dropped, a deliberate double press is not), two quick candidate clicks on the rail, and since #386 a double-press of P.

Possible directions

• An in-flight guard on the editor's commit path (drop commits while one is pending), or
• upsert semantics on the single-annotation POST (the bulk endpoint already guards this case), or
• an optimistic cache write so the second commit sees the annotation and PATCHes.

Start at LocalizeAlertPage and trace saveDetection.onSuccess, saveDetectionReview, and the useMutation.mutate call through the editor commit path. Reproduce two fast commits on an undecided frame and compare the available guard, upsert, and optimistic-cache directions. Done means the duplicate request no longer produces a failure toast when the first save succeeded.