feat(dashboard): tiles for HRV, blood oxygen, breathing rate, temperature, and body composition

Several readings were synced and charted but had no place on the dashboard
strip: heart-rate variability, blood oxygen, breathing rate, wrist temperature,
and — for body-composition scale users — muscle mass, body water, and bone mass.
Each now has a tile.

The tiles are pure client additions: the dashboard snapshot already ships a
summary for every measurement type, so no backend, query, or migration changed.
Each tile is data-availability gated (renders only when the account has that
reading) and is toggleable and reorderable under Settings like the rest.

hrv and respiratoryRate graduate from iOS-only to web-writable widget ids, and
muscleMass is a new writable id (catalogue 36 to 37, within the 40-id PUT cap);
wristTemperature moves from pin-only to a web-rendered tile. All additive — the
widget enum only widens, which the iOS contract treats as non-breaking.

docs/api/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: HealthLog API
-  version: 1.28.51
+  version: 1.28.52
   description: >-
     Self-hosted personal-health-tracking PWA — public API surface for the iOS native client and external ingest.