Optimize: Fix high/medium/low priority issues and apply optimizations

High Priority Fixes:
- Use configurable ClusterTimeoutMinutes for restore (was hardcoded 2 hours)
- Add comment explaining goroutine cleanup in stderr reader (cmd.Run waits)
- Add defer cancel() in cluster backup loop to prevent context leak on panic

Medium Priority Fixes:
- Standardize tick rate to 100ms for both backup and restore (consistent UX)
- Add spinnerFrame field to BackupExecutionModel for incremental updates
- Define package-level spinnerFrames constant to avoid repeated allocation

Low Priority Fixes:
- Add 30-second timeout per database in cluster cleanup loop
- Prevents indefinite hangs when dropping many databases

Optimizations:
- Pre-allocate 512 bytes in View() string builders (reduces allocations)
- Use incremental spinner frame calculation (more efficient than time-based)
- Share spinner frames array across all TUI operations

All changes are backward compatible and maintain existing behavior.

internal/tui/backup_exec.go

@@ -29,6 +29,7 @@ type BackupExecutionModel struct {
 	result       string
 	startTime    time.Time
 	details      []string
+	spinnerFrame int
 }
 
 func NewBackupExecution(cfg *config.Config, log logger.Logger, parent tea.Model, backupType, dbName string, ratio int) BackupExecutionModel {
@@ -42,6 +43,7 @@ func NewBackupExecution(cfg *config.Config, log logger.Logger, parent tea.Model,
 		status:       "Initializing...",
 		startTime:    time.Now(),
 		details:      []string{},
+		spinnerFrame: 0,
 	}
 }
 
@@ -144,6 +146,9 @@ func (m BackupExecutionModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 	switch msg := msg.(type) {
 	case backupTickMsg:
 		if !m.done {
+			// Increment spinner frame for smooth animation
+			m.spinnerFrame = (m.spinnerFrame + 1) % len(spinnerFrames)
+			
 			// Update status based on elapsed time to show progress
 			elapsedSec := int(time.Since(m.startTime).Seconds())
 			
@@ -207,6 +212,7 @@ func (m BackupExecutionModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 
 func (m BackupExecutionModel) View() string {
 	var s strings.Builder
+	s.Grow(512) // Pre-allocate estimated capacity for better performance
 
 	// Clear screen with newlines and render header
 	s.WriteString("\n\n")
@@ -227,9 +233,7 @@ func (m BackupExecutionModel) View() string {
 
 	// Status with spinner
 	if !m.done {
-		spinner := []string{"⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏"}
-		frame := int(time.Since(m.startTime).Milliseconds()/100) % len(spinner)
-		s.WriteString(fmt.Sprintf("  %s %s\n", spinner[frame], m.status))
+		s.WriteString(fmt.Sprintf("  %s %s\n", spinnerFrames[m.spinnerFrame], m.status))
 	} else {
 		s.WriteString(fmt.Sprintf("  %s\n\n", m.status))