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/superpowers/plans/2026-07-26-onboarding-wizard-deferred-commit.md

@@ -368,7 +368,7 @@ import (
 )
 ```
 
-Add the `setupGarmin` field to `Server` and initialize it in `NewServer`:
+Add the `setupSession` field to `Server` and initialize it in `NewServer`:
 
 ```go
 	mu              sync.Mutex
@@ -395,7 +395,7 @@ Add the `setupGarmin` field to `Server` and initialize it in `NewServer`:
 }
 ```
 
-Insert this block right after `NewServer` (before `garminFor`):
+Insert this block right after `NewServer` (before `clientFor`):
 
 ```go