refactor(config): rename default Garmin token-store dir to .garmin

Shorter, more conventional dotdir name than garmin-tokenstores. Restore
the "next to DBPath" join that got dropped when the default was
inlined via getEnvDefault -- an explicit GARMIN_TOKENSTORE still wins
verbatim, but the implicit default must still resolve relative to the
DB file's directory, not the process's CWD.

internal/garmin/client.go's GARMIN_TOKENSTORE env var is now always
set (TokenStorePath can no longer be empty), so the conditional that
only appended it when non-empty is dead code.

backend/internal/config/config_test.go

@@ -88,7 +88,7 @@ func TestLoad_GarminTokenStoreRootDefaultsNextToDBPath(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Load: %v", err)
 	}
-	want := filepath.Join("/var/lib/geniusrun", "garmin-tokenstores")
+	want := filepath.Join("/var/lib/geniusrun", ".garmin")
 	if cfg.GarminTokenStoreRoot != want {
 		t.Errorf("GarminTokenStoreRoot = %q, want %q", cfg.GarminTokenStoreRoot, want)
 	}