Stage 1: project setup, Exercise Library, both Mode designs and the Mode switch

- Flutter Android app on CookIT's stack (provider, shared_preferences, gen-l10n)
- tools/exercise-import turns free-exercise-db into 876 bundled Exercises
  with 1,746 WebP images (20 MB); stretching -> Stretching, rest -> Workout
- ZenStyle tokens per Mode: calm rounded sage/teal with breath ripples vs.
  dark ember-to-red with floor-tape slashes; the theme morphs at each
  Mode's own pace
- Remembered global Mode switch, Plans / Library / History tabs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/src/localization/app_en.arb

@@ -0,0 +1,29 @@
+{
+  "@@locale": "en",
+  "appTitle": "ZenXercise",
+  "modeStretching": "Stretching",
+  "modeWorkout": "Workout",
+  "modeSwitchLabel": "Mode",
+  "navPlans": "Plans",
+  "navLibrary": "Library",
+  "navHistory": "History",
+  "plansTitle": "Your plans",
+  "plansEmptyTitle": "No plans yet",
+  "plansEmptyBody": "{mode, select, stretching{Put together a stretch routine from the library and it will wait for you here.} other{Put together a workout from the library and it will wait for you here.}}",
+  "@plansEmptyBody": {
+    "placeholders": {
+      "mode": {"type": "String"}
+    }
+  },
+  "libraryTitle": "Library",
+  "librarySubtitle": "{mode, select, stretching{{count, plural, =1{1 stretch} other{{count} stretches}}} other{{count, plural, =1{1 exercise} other{{count} exercises}}}}",
+  "@librarySubtitle": {
+    "placeholders": {
+      "mode": {"type": "String"},
+      "count": {"type": "int"}
+    }
+  },
+  "historyTitle": "History",
+  "historyEmptyTitle": "Nothing logged yet",
+  "historyEmptyBody": "Finished sessions show up here with their date and duration."
+}