Implement per-host widget cache for instant host switching

Resolves widget data persistence issue where switching hosts left stale data
from the previous host displayed in widgets.

Key improvements:
- Add Clone derives to all widget structs (CpuWidget, MemoryWidget,
  ServicesWidget, BackupWidget)
- Create HostWidgets struct to cache widget states per hostname
- Update TuiApp with HashMap<String, HostWidgets> for per-host storage
- Fix borrowing issues by cloning hostname before mutable self borrow
- Implement instant widget state restoration when switching hosts

Tab key host switching now displays cached widget data for each host
without stale information persistence between switches.

dashboard/src/ui/widgets/backup.rs

@@ -10,6 +10,7 @@ use super::Widget;
 use crate::ui::theme::{Theme, Typography, Components, StatusIcons};
 
 /// Backup widget displaying backup status, services, and repository information
+#[derive(Clone)]
 pub struct BackupWidget {
     /// Overall backup status
     overall_status: Status,

dashboard/src/ui/widgets/cpu.rs

@@ -10,6 +10,7 @@ use super::Widget;
 use crate::ui::theme::{Theme, Typography, Components, StatusIcons};
 
 /// CPU widget displaying load, temperature, and frequency
+#[derive(Clone)]
 pub struct CpuWidget {
     /// CPU load averages (1, 5, 15 minutes)
     load_1min: Option<f32>,

dashboard/src/ui/widgets/memory.rs

@@ -10,6 +10,7 @@ use super::Widget;
 use crate::ui::theme::{Theme, Typography, Components, StatusIcons};
 
 /// Memory widget displaying usage, totals, and swap information
+#[derive(Clone)]
 pub struct MemoryWidget {
     /// Memory usage percentage
     usage_percent: Option<f32>,

dashboard/src/ui/widgets/services.rs

@@ -12,6 +12,7 @@ use crate::ui::theme::{Theme, Typography, Components, StatusIcons};
 use ratatui::style::Style;
 
 /// Services widget displaying hierarchical systemd service statuses
+#[derive(Clone)]
 pub struct ServicesWidget {
     /// Parent services (nginx, docker, etc.)
     parent_services: HashMap<String, ServiceInfo>,