fix(forms): send one Idempotency-Key per opened create form

A mood, symptom, lab value, custom metric value, unscheduled dose,
allergy, vaccination, visit or measurement saved on a weak signal could
be stored twice: the POST committed, the answer missed the 15 s
timeout, the form said it could not save, and the second tap was a new
call with a new key, so the server ran it again.

useIdempotentPost keeps one key until a save lands and renews it for the
next entry, so the retry replays the first answer. The create forms and
the three create hooks post through it; the comment on the per-call key
in api-fetch now says what that key does and does not cover.

LabForm and MoodForm each gain the one line that mounts the key, so
their function-size baselines go up by one. The intake toast guard now
recognises a dose posted through the hook.

src/__tests__/baselines/source-function-lines.json

@@ -134,7 +134,7 @@
     "components/labs/biomarker-merge-sheet.tsx :: BiomarkerMergeSheet": 239,
     "components/labs/lab-biomarker-chart.tsx :: LabBiomarkerChart": 370,
     "components/labs/lab-biomarker-detail.tsx :: LabBiomarkerDetail": 481,
-    "components/labs/lab-form.tsx :: LabForm": 473,
+    "components/labs/lab-form.tsx :: LabForm": 474,
     "components/labs/lab-history-list.tsx :: LabHistoryList": 534,
     "components/labs/ocr-review-dialog.tsx :: OcrReviewDialog": 359,
     "components/labs/ocr-row-editor.tsx :: OcrRowEditor": 378,
@@ -173,7 +173,7 @@
     "components/mental-health/mental-wellbeing.tsx :: MentalWellbeing": 202,
     "components/mood/manage/tag-groups-card.tsx :: TagGroupsCard": 256,
     "components/mood/manage/tag-manager-card.tsx :: TagManagerCard": 270,
-    "components/mood/mood-form.tsx :: MoodForm": 513,
+    "components/mood/mood-form.tsx :: MoodForm": 514,
     "components/mood/mood-list.tsx :: MoodList": 1109,
     "components/mood/mood-tag-picker.tsx :: MoodTagPicker": 257,
     "components/onboarding/confirm-screen.tsx :: ConfirmScreen": 255,