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/settings.go

@@ -115,6 +115,26 @@ func NewSettingsModel(cfg *config.Config, log logger.Logger, parent tea.Model) S
 			Type:        "path",
 			Description: "Directory where backup files will be stored",
 		},
+		{
+			Key:         "work_dir",
+			DisplayName: "Work Directory",
+			Value: func(c *config.Config) string {
+				if c.WorkDir == "" {
+					return "(system temp)"
+				}
+				return c.WorkDir
+			},
+			Update: func(c *config.Config, v string) error {
+				if v == "" || v == "(system temp)" {
+					c.WorkDir = ""
+					return nil
+				}
+				c.WorkDir = filepath.Clean(v)
+				return nil
+			},
+			Type:        "path",
+			Description: "Working directory for large operations (extraction, diagnosis). Use when /tmp is too small.",
+		},
 		{
 			Key:         "compression_level",
 			DisplayName: "Compression Level",