fix: resolve Gemini review findings — race conditions and error handling

- error.go: appendReport now mutex-protected, handles JSON errors, uses 0600 perms
- log.go: keyvals slice copied before mutation to prevent caller data races
- log.go: defaultLog uses atomic.Pointer for thread-safe replacement

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/core/error.go

@@ -16,6 +16,7 @@ import (
 	"runtime"
 	"runtime/debug"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -390,14 +391,21 @@ func (h *ErrPan) Reports(n int) ([]CrashReport, error) {
 	return reports[len(reports)-n:], nil
 }
 
-func (h *ErrPan) appendReport(report CrashReport) {
-	var reports []CrashReport
+var crashMu sync.Mutex
 
+func (h *ErrPan) appendReport(report CrashReport) {
+	crashMu.Lock()
+	defer crashMu.Unlock()
+
+	var reports []CrashReport
 	if data, err := os.ReadFile(h.filePath); err == nil {
-		json.Unmarshal(data, &reports)
+		if err := json.Unmarshal(data, &reports); err != nil {
+			reports = nil
+		}
 	}
 
 	reports = append(reports, report)
-	data, _ := json.MarshalIndent(reports, "", "  ")
-	os.WriteFile(h.filePath, data, 0644)
+	if data, err := json.MarshalIndent(reports, "", "  "); err == nil {
+		_ = os.WriteFile(h.filePath, data, 0600)
+	}
 }

pkg/core/log.go

@@ -12,6 +12,7 @@ import (
 	"os/user"
 	"slices"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -175,6 +176,9 @@ func (l *Log) log(level Level, prefix, msg string, keyvals ...any) {
 
 	timestamp := styleTimestamp(time.Now().Format("15:04:05"))
 
+	// Copy keyvals to avoid mutating the caller's slice
+	keyvals = append([]any(nil), keyvals...)
+
 	// Automatically extract context from error if present in keyvals
 	origLen := len(keyvals)
 	for i := 0; i < origLen; i += 2 {
@@ -294,16 +298,23 @@ func Username() string {
 
 // --- Default logger ---
 
-var defaultLog = NewLog(LogOpts{Level: LevelInfo})
+var defaultLogPtr atomic.Pointer[Log]
+
+func init() {
+	l := NewLog(LogOpts{Level: LevelInfo})
+	defaultLogPtr.Store(l)
+}
+
+var defaultLog = defaultLogPtr.Load()
 
 // Default returns the default logger.
 func Default() *Log {
-	return defaultLog
+	return defaultLogPtr.Load()
 }
 
-// SetDefault sets the default logger.
+// SetDefault sets the default logger (thread-safe).
 func SetDefault(l *Log) {
-	defaultLog = l
+	defaultLogPtr.Store(l)
 }
 
 // SetLevel sets the default logger's level.