feat: support updating Garmin credentials at runtime

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

backend/internal/garmin/client.go

@@ -24,6 +24,12 @@ type Client interface {
 	Authenticate(ctx context.Context) (AuthResult, error)
 	// CompleteMFA submits an MFA code for a login started by Authenticate.
 	CompleteMFA(ctx context.Context, code string) (AuthResult, error)
+	// UpdateCredentials replaces the Garmin email/password used to spawn
+	// the subprocess, and terminates any already-running subprocess (which
+	// would otherwise still be authenticated under the old credentials).
+	// The next call that needs the subprocess spawns a fresh one with the
+	// new credentials.
+	UpdateCredentials(email, password string)
 	// GetActivities lists activities between start and end (YYYY-MM-DD).
 	GetActivities(ctx context.Context, startDate, endDate string, limit int) ([]Activity, error)
 	// GetActivitySplits fetches lap/split summaries for one activity.
@@ -97,6 +103,23 @@ func (c *mcpClient) ensureStarted(ctx context.Context) error {
 	return nil
 }
 
+// UpdateCredentials implements Client.
+func (c *mcpClient) UpdateCredentials(email, password string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.cfg.GarminEmail = email
+	c.cfg.GarminPassword = password
+
+	if c.started {
+		if c.inner != nil {
+			c.inner.Close()
+		}
+		c.inner = nil
+		c.started = false
+	}
+}
+
 // drainStderr forwards the subprocess's debug/log output so it isn't
 // silently dropped (mcp-garmin logs auth/rate-limit diagnostics there).
 func drainStderr(stdio *transport.Stdio) {

backend/internal/garmin/client_test.go

@@ -20,3 +20,20 @@ func TestParseAuthResult(t *testing.T) {
 		}
 	}
 }
+
+func TestMcpClient_UpdateCredentials_ResetsStartedState(t *testing.T) {
+	c := &mcpClient{cfg: Config{GarminEmail: "old@example.com", GarminPassword: "old"}}
+	c.started = true // simulate an already-spawned subprocess
+
+	c.UpdateCredentials("new@example.com", "new")
+
+	if c.cfg.GarminEmail != "new@example.com" || c.cfg.GarminPassword != "new" {
+		t.Errorf("cfg after update = %+v, want new@example.com/new", c.cfg)
+	}
+	if c.started {
+		t.Error("started should be reset to false so the next call respawns the subprocess")
+	}
+	if c.inner != nil {
+		t.Error("inner should be cleared so ensureStarted spawns a fresh client")
+	}
+}

backend/internal/garmin/mock/mock.go

@@ -18,6 +18,8 @@ type Client struct {
 	authResultCursor   int
 	ClosedCalled       bool
 	GetActivitiesCalls int
+	LastEmail          string
+	LastPassword       string
 }
 
 var _ garmin.Client = (*Client)(nil)
@@ -70,6 +72,11 @@ func (c *Client) GetActivityDetails(ctx context.Context, activityID int64) (garm
 	return c.Details[activityID], nil
 }
 
+func (c *Client) UpdateCredentials(email, password string) {
+	c.LastEmail = email
+	c.LastPassword = password
+}
+
 func (c *Client) Close() error {
 	c.ClosedCalled = true
 	return nil