feat(ui): tabs folding into the title, in Learn's and Train's Profile (ADR 0030, D60)

halkyon_ui's FoldingTabs: the tab row under the app bar's title at the
top, scrolling away with the content, the chosen tab's name then small
under the title, which, tapped, scrolls back up to the row, as Strava's.
Learn's Profile (Profile, Statistics) is built on it, and Train's
Profile in tabs: Progress (your body, the last 12 weeks, this year),
Days, and Plans where the device doesn't record.

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

@@ -312,6 +312,7 @@
   "loopFormIntro": "Your band needs these. They're kept in Your body, sealed with your key. For a resting or highest heart rate or a VO2max you haven't given, the band takes Polar's defaults, and Your body stays as it is.",
   "loopFormSave": "Set up the band",
   "homeWorkouts": "Workouts",
+  "profileTabProgress": "Progress",
   "homeDays": "Days",
   "homePlans": "Plans",
   "plansNew": "New plan",