fix(forms): keep one Idempotency-Key for the remaining create forms

A screening, an illness episode, a life event, a symptom, a custom
metric or an address-book entry created on a weak signal could be
stored twice: the POST committed, the answer missed the timeout, and
the second tap went out under a new key. Their routes already honour
the key; the forms now post through useIdempotentPost like the other
create forms, so the retry replays the first answer.

The screening's save moves into a small hook beside MentalWellbeing,
which sat at the function-size cap and now drops below it; the custom
metric form's create-or-update call moves into one as well.

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

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
-  "count": 339,
-  "total": 110086,
+  "count": 338,
+  "total": 109885,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 275,
@@ -168,7 +168,6 @@
     "components/medications/wizard/medication-wizard-dialog.tsx :: WizardDialogShell": 516,
     "components/medications/wizard/steps/step3-dose.tsx :: Step3Dose": 222,
     "components/medications/wizard/steps/step8-summary.tsx :: Step8Summary": 230,
-    "components/mental-health/mental-wellbeing.tsx :: MentalWellbeing": 201,
     "components/mood/manage/tag-groups-card.tsx :: TagGroupsCard": 256,
     "components/mood/manage/tag-manager-card.tsx :: TagManagerCard": 270,
     "components/mood/mood-form.tsx :: MoodForm": 514,