refactor(checkups): split the checkups section into form, card and postpone sheet

The checkups file held two components of about 620 lines each. Their
parts move to components/measurement-reminders/checkups/:

- reminder-form: the type groups, cadence presets, the form state and
  its mapping to and from a reminder (pure)
- reminder-form-sheet: the create / edit sheet and its cadence fields
- checkup-card: the card in both views, its menu, details, skip and
  snooze lines, next / last lines and action row
- postpone-sheet: the postpone chips and date, the honest skip and its
  confirmation, and the calendar-day offsets

vorsorge-section.tsx keeps the list, the value capture and the visit
filing. Markup and behaviour are unchanged; the guards name the modules
that now hold what they check.

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": 402,
-  "total": 138967,
+  "count": 401,
+  "total": 138066,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -177,8 +177,7 @@
     "components/layout/top-bar.tsx :: TopBar": 212,
     "components/links/entity-link-picker.tsx :: EntityLinkPicker": 392,
     "components/measurement-reminders/vorsorge-dashboard-card.tsx :: VorsorgeDashboardCard": 215,
-    "components/measurement-reminders/vorsorge-section.tsx :: VorsorgeCard": 635,
-    "components/measurement-reminders/vorsorge-section.tsx :: VorsorgeSection": 618,
+    "components/measurement-reminders/vorsorge-section.tsx :: VorsorgeSection": 352,
     "components/measurements/measurement-form.tsx :: MeasurementForm": 500,
     "components/medications/detail/efficacy/efficacy-chart.tsx :: EfficacyChart": 361,
     "components/medications/detail/medication-detail-tabs.tsx :: MedicationDetailTabs": 771,