fix(garmin): wait for stderr-copy goroutine before cmd.Wait()

ensureStarted's stderr-copy goroutine could still be mid-Read when close()
called cmd.Wait(), which os/exec's StderrPipe docs call out as incorrect
and can truncate/garble trailing stderr diagnostics or surface a spurious
"file already closed" error. close() now waits on a stderrDone channel,
closed by the copy goroutine once it hits EOF (unblocked by killing the
process), before calling Wait.

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

backend/internal/garmin/client.go

@@ -116,6 +116,7 @@ type subprocessClient struct {
 	scriptPath string // temp file holding the embedded wrapper.py, written once
 	started    bool
 	nextID     int
+	stderrDone chan struct{} // closed once the stderr-copy goroutine has finished reading
 }
 
 // ensureStarted spawns the wrapper subprocess if it isn't already running.
@@ -172,7 +173,15 @@ func (c *subprocessClient) ensureStarted() error {
 	if err := cmd.Start(); err != nil {
 		return fmt.Errorf("spawn garmin wrapper subprocess: %w", err)
 	}
-	go io.Copy(os.Stderr, stderr) // wrapper.py logs auth/rate-limit diagnostics to stderr
+	// wrapper.py logs auth/rate-limit diagnostics to stderr. Per os/exec's
+	// StderrPipe docs, it's incorrect to call Wait before all reads from the
+	// pipe have completed, so close() waits on stderrDone before Wait-ing.
+	c.stderrDone = make(chan struct{})
+	stderrDone := c.stderrDone
+	go func() {
+		io.Copy(os.Stderr, stderr)
+		close(stderrDone)
+	}()
 
 	c.cmd = cmd
 	c.stdin = stdin
@@ -246,11 +255,17 @@ func (c *subprocessClient) close() error {
 	if c.cmd != nil && c.cmd.Process != nil {
 		c.cmd.Process.Kill()
 	}
+	if c.stderrDone != nil {
+		// Killing the process closes its end of the stderr pipe, which
+		// unblocks the copy goroutine's Read with EOF; wait for it to finish
+		// before Wait, per os/exec's StderrPipe doc.
+		<-c.stderrDone
+	}
 	var err error
 	if c.cmd != nil {
 		err = c.cmd.Wait()
 	}
-	c.cmd, c.stdin, c.enc, c.scanner = nil, nil, nil, nil
+	c.cmd, c.stdin, c.enc, c.scanner, c.stderrDone = nil, nil, nil, nil, nil
 	return err
 }