refactor(profiles): replace large-db profile with composable LargeDBMode

BREAKING CHANGE: Removed 'large-db' as standalone profile

New Design:
- Resource Profiles now purely represent VM capacity:
  conservative, balanced, performance, max-performance
- LargeDBMode is a separate boolean toggle that modifies any profile:
  - Reduces ClusterParallelism and Jobs by 50%
  - Forces MaxLocksPerTxn = 8192
  - Increases MaintenanceWorkMem

TUI Changes:
- 'l' key now toggles LargeDBMode ON/OFF instead of applying large-db profile
- New 'Large DB Mode' setting in settings menu
- Settings are persisted to .dbbackup.conf

This allows any resource profile to be combined with large database
optimization, giving users more flexibility on both small and large VMs.

internal/tui/restore_preview.go

@@ -410,6 +410,10 @@ func (m RestorePreviewModel) View() string {
 		} else {
 			s.WriteString(fmt.Sprintf("  Resource Profile: %s\n", m.config.ResourceProfile))
 		}
+		// Show Large DB Mode status
+		if m.config.LargeDBMode {
+			s.WriteString("  Large DB Mode: ON (reduced parallelism, high locks)\n")
+		}
 		s.WriteString(fmt.Sprintf("  CPU Workload: %s\n", m.config.CPUWorkloadType))
 		s.WriteString(fmt.Sprintf("  Cluster Parallelism: %d databases\n", m.config.ClusterParallelism))