feat(train): the Loop's training sessions as workouts (ADR 0030, D39)

Each read of the band lists the sessions it holds, reads those from the
read's first day on (`LoopDays.kt`, one line per exercise: its start
with the band's offset, length, Polar's sport ID, figures and heart
rate at its interval), and deletes those up to the day kept, as the
days. The app keeps each as a workout of origin Polar Loop with its
heart rate (`takeLoopSessions`), Polar's running and cycling as
Train's and every other sport as the new `Activity.other`, which a
workout's edit sheet now changes; a session overlapping a workout Train
recorded gives it its heart rate where it has none instead. The
sessions taken are remembered in `loop.json`, so one deleted in Train
isn't brought back. Health Connect writes the Loop's workouts as
Train's own.

The SDK gives sessions as its protobuf messages: protobuf-javalite
3.20.0, the SDK's own, is named for compiling only.

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_en.arb

@@ -25,6 +25,7 @@
   "activityHiking": "Hiking",
   "activityCycling": "Cycling",
   "activityIndoorRowing": "Rowing indoors",
+  "activityOther": "Other workout",
   "recordAutoPause": "Pause by itself",
   "@recordAutoPause": {"description": "The switch for auto-pause, as the recording starts."},
   "recordAutoPauseHint": "When you stand still, and going on as you move again",
@@ -75,6 +76,7 @@
   "workoutEdit": "Edit",
   "workoutEditTitle": "Edit workout",
   "workoutNameLabel": "Name",
+  "workoutActivityLabel": "Activity",
   "workoutNotesLabel": "Notes",
   "workoutSave": "Save",
   "workoutDelete": "Delete",