fix(ui): stack the date-time field on narrow viewports

A native date input keeps its intrinsic width on mobile WebKit and won't
shrink, so the `date + fixed-width time` row overran a phone-width sheet —
the time field and its clock icon spilled past the screen edge and every
add form (measurements, mood, medication, …) scrolled sideways. Stack the
two halves on narrow viewports and only place them side-by-side from the
`sm` breakpoint up; each half then takes the full sheet width and nothing
overflows.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.25.9
+  version: 1.25.10
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.