Remove service restart functionality and make R always rebuild host

Simplified keyboard controls by removing service restart functionality:

- Removed 'r' key restart functionality from Services panel
- Made 'R' key always trigger system rebuild regardless of focused panel
- Updated context shortcuts to show 'R: Rebuild Host' globally
- Removed all ServiceRestart enum variants and associated code:
  - UiCommand::ServiceRestart
  - CommandType::ServiceRestart
  - ServiceAction::Restart
- Cleaned up pending transition logic to only handle Start/Stop commands

The 'R' key now consistently rebuilds the current host from any panel,
while 's' and 'S' continue to handle service start/stop in Services panel.

dashboard/src/communication/mod.rs

@@ -35,7 +35,6 @@ pub enum AgentCommand {
 pub enum ServiceAction {
     Start,
     Stop,
-    Restart,
     Status,
 }