refactor(auth): Centralize auth file reading with snapshot preference

The logic for reading authentication files, which includes retries and a preference for cookie snapshot files, was previously implemented locally within the `watcher` package. This was done to handle potential file locks during writes.

This change moves this functionality into a shared `ReadAuthFileWithRetry` function in the `util` package to promote code reuse and consistency.

The `watcher` package is updated to use this new centralized function. Additionally, the initial token loading in the `run` command now also uses this logic, making it more resilient to file access issues and consistent with the watcher's behavior.

internal/util/cookie_snapshot.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"github.com/luispater/CLIProxyAPI/v5/internal/misc"
 )
@@ -93,6 +94,52 @@ func WriteCookieSnapshot(mainPath string, v any) error {
 	return nil
 }
 
+// ReadAuthFilePreferSnapshot returns the first non-empty auth payload preferring snapshots.
+func ReadAuthFilePreferSnapshot(path string) ([]byte, error) {
+	return ReadAuthFileWithRetry(path, 1, 0)
+}
+
+// ReadAuthFileWithRetry attempts to read an auth file multiple times and prefers cookie snapshots.
+func ReadAuthFileWithRetry(path string, attempts int, delay time.Duration) ([]byte, error) {
+	if attempts < 1 {
+		attempts = 1
+	}
+	read := func(target string) ([]byte, error) {
+		var lastErr error
+		for i := 0; i < attempts; i++ {
+			data, err := os.ReadFile(target)
+			if err == nil {
+				return data, nil
+			}
+			lastErr = err
+			if i < attempts-1 {
+				time.Sleep(delay)
+			}
+		}
+		return nil, lastErr
+	}
+
+	candidates := []string{
+		CookieSnapshotPath(path),
+		path,
+	}
+
+	for idx, candidate := range candidates {
+		data, err := read(candidate)
+		if err == nil {
+			return data, nil
+		}
+		if errors.Is(err, os.ErrNotExist) {
+			if idx < len(candidates)-1 {
+				continue
+			}
+		}
+		return nil, err
+	}
+
+	return nil, os.ErrNotExist
+}
+
 // RemoveCookieSnapshots removes the snapshot file if it exists.
 func RemoveCookieSnapshots(mainPath string) {
 	_ = RemoveFile(CookieSnapshotPath(mainPath))