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/garmin/client.go

@@ -63,8 +63,8 @@ type Config struct {
 	PythonPath     string
 	GarminEmail    string
 	GarminPassword string
-	// TokenStorePath, if set, is passed as GARMIN_TOKENSTORE so the wrapper
-	// persists/resumes a Garmin session there instead of the default ~/.garth.
+	// TokenStorePath is passed as GARMIN_TOKENSTORE so the wrapper
+	// persists/resumes a Garmin session there.
 	TokenStorePath string
 }
 
@@ -155,11 +155,9 @@ func (c *subprocessClient) ensureStarted(ctx context.Context) error {
 	extraEnv := []string{
 		"GARMIN_EMAIL=" + c.cfg.GarminEmail,
 		"GARMIN_PASSWORD=" + c.cfg.GarminPassword,
+		"GARMIN_TOKENSTORE=" + c.cfg.TokenStorePath,
 		"PYTHONUNBUFFERED=1",
 	}
-	if c.cfg.TokenStorePath != "" {
-		extraEnv = append(extraEnv, "GARMIN_TOKENSTORE="+c.cfg.TokenStorePath)
-	}
 	cmd.Env = append(os.Environ(), extraEnv...)
 
 	stdin, err := cmd.StdinPipe()
@@ -199,7 +197,6 @@ func (c *subprocessClient) ensureStarted(ctx context.Context) error {
 
 	applog.FromContext(ctx).Info("garmin wrapper spawning",
 		"python_path", pythonPath,
-		"token_store_configured", c.cfg.TokenStorePath != "",
 	)
 	return nil
 }