feat(tui): add Work Directory setting for large archive operations

- Added WorkDir to Config for custom temp directory
- TUI Settings: new 'Work Directory' option to set alternative temp location
- Restore Preview: press 'w' to toggle work directory (uses backup dir as default)
- Diagnose View: now uses configured WorkDir for cluster extraction
- Config persistence: WorkDir saved to .dbbackup.conf

This fixes diagnosis/restore failures when /tmp is too small for large archives.
Use cases: servers with limited /tmp, 70GB+ archives needing 280GB+ extraction space.

internal/tui/diagnose_view.go

@@ -88,8 +88,8 @@ func runDiagnosis(cfg *config.Config, log logger.Logger, archive ArchiveInfo) te
 
 		// For cluster archives, we can do deep analysis
 		if archive.Format.IsClusterBackup() {
-			// Create temp directory
-			tempDir, err := createTempDir("dbbackup-diagnose-*")
+			// Create temp directory (use WorkDir if configured for large archives)
+			tempDir, err := createTempDirIn(cfg.WorkDir, "dbbackup-diagnose-*")
 			if err != nil {
 				return diagnoseCompleteMsg{err: fmt.Errorf("failed to create temp dir: %w", err)}
 			}
@@ -445,6 +445,17 @@ func createTempDir(pattern string) (string, error) {
 	return os.MkdirTemp("", pattern)
 }
 
+func createTempDirIn(baseDir, pattern string) (string, error) {
+	if baseDir == "" {
+		return os.MkdirTemp("", pattern)
+	}
+	// Ensure base directory exists
+	if err := os.MkdirAll(baseDir, 0755); err != nil {
+		return "", fmt.Errorf("cannot create work directory: %w", err)
+	}
+	return os.MkdirTemp(baseDir, pattern)
+}
+
 func removeTempDir(path string) error {
 	return os.RemoveAll(path)
 }