feat(train): a workout out as GPX, TCX or FIT (ADR 0030, D49)

11b: Export on a workout's page hands it to the share sheet as GPX (with
a track only; heart rate and cadence in Garmin's TrackPointExtension),
TCX (laps, heart rate, cadence, watts, a row's distance) or FIT (file
ID, records, timer events at the pauses, laps with their intensities,
session, activity), all from the same points: the precise fixes while
moving, else the rower's frames, else the heart rates; a plan's steps
are the laps. The FIT writer is our own (protocol 1.0, CRC-16/ARC),
tested by a reader written from the specification.

Not tried yet with Strava, intervals.icu or Garmin Connect.

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

@@ -653,5 +653,10 @@
   "announceSpeed": "Speed {speed} kilometres an hour",
   "@announceSpeed": {"placeholders": {"speed": {"type": "String"}}},
   "announceHeartRate": "Heart rate {bpm}",
-  "@announceHeartRate": {"placeholders": {"bpm": {"type": "String"}}}
+  "@announceHeartRate": {"placeholders": {"bpm": {"type": "String"}}},
+  "workoutExport": "Export",
+  "workoutExportTitle": "Export the workout",
+  "workoutExportGpxHint": "The track, for maps and other apps",
+  "workoutExportTcxHint": "Laps, heart rate and cadence, for most training sites",
+  "workoutExportFitHint": "What Strava, Garmin and intervals.icu read best"
 }