feat(train): the heart-rate strap, recorded by the service itself (ADR 0030, D25-D27)

The Record sheet gains a Heart rate row: a sheet looks for straps
advertising the Heart Rate Service, the nearest first, once nearby
devices are allowed, and the one chosen is remembered in the person's
folder; it records with every workout unless switched off for one.
The recorder service connects to it with Android's own Bluetooth API,
no package (D26), parses Heart Rate Measurement in Kotlin (tested on
the JVM), writes each rate and the strap's state to the recording
file, and asks for a lost strap again for as long as it records. The
recording screen shows the heart rate, and "Strap lost" while it is
gone; a workout's page shows the heart's average and highest rate,
each split's average and its chart.

iOS's recording waits for a Mac (D25), after the rower.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/lib/l10n/app_de.arb

@@ -82,5 +82,23 @@
   "workoutMapSource": "Karte von {name}: sieht die Adresse deines Geräts und den gezeigten Ausschnitt, nicht deine Strecke.",
   "workoutMapSourceAbout": "Was {name} speichert",
   "workoutMapUnavailable": "Die Karte kann gerade nicht geladen werden.",
-  "workoutNotFound": "Dieses Training gibt es nicht mehr."
+  "workoutNotFound": "Dieses Training gibt es nicht mehr.",
+
+  "recordHeartRate": "Herzfrequenz",
+  "recordNoStrap": "Kein Brustgurt",
+  "strapSheetTitle": "Brustgurt",
+  "strapSearching": "Suche Brustgurte in der Nähe …",
+  "strapSearchingHint": "Leg den Gurt an: Die meisten wachen erst auf, wenn sie die Haut berühren.",
+  "strapNear": "ganz nah",
+  "strapFar": "weiter weg",
+  "strapUnnamed": "Brustgurt ohne Namen",
+  "strapForget": "Brustgurt vergessen",
+  "strapRefused": "Train braucht Geräte in der Nähe, um deinen Brustgurt zu finden. Du kannst das in den Systemeinstellungen erlauben.",
+  "strapBluetoothOff": "Bluetooth ist aus. Schalte es ein, um deinen Brustgurt zu finden.",
+  "recordingHeartRate": "Herzfrequenz",
+  "recordingStrapLost": "Brustgurt nicht verbunden",
+  "workoutAverageHeartRate": "mittlere Herzfrequenz",
+  "workoutMaxHeartRate": "höchste Herzfrequenz",
+  "workoutBpmValue": "{bpm} bpm",
+  "workoutHeartRateChart": "Herzfrequenz"
 }