refactor: merge internal/sync into internal/garmin, regroup api files and routes

Garmin auth/sync routes move under /api/garmin/*; sync.Service becomes
garmin.Sync with garmin.SyncConfig/ClientConfig; applog becomes
internal/log; the test mock moves into the garmin package as MockClient
(breaking the test-only import cycle the merge created); stale test
URLs and type names updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/workouts.yaml

@@ -0,0 +1,12 @@
+version: v1
+name: geniusrun
+templates:
+  - name: 'Recovery 30m'
+    difficulty: 1
+    kind: recovery
+    blocks:
+      - index: 1
+        reps: 1
+        block:
+          - kind: run
+            duration: 30m