Add hostname editing feature to agent detail page

- Create enhanced agent detail template with all stats and hostname edit form
- Add UpdateHostname method to store for agent hostname updates
- Add UpdateAgentHostname handler in UI for web form submissions
- Add UpdateHostname endpoint in API handler for JSON requests
- Register new POST /agents/{id}/hostname route for hostname updates
- Improve agent detail page layout with Settings and Danger Zone sections
- Add error handling for missing agents and empty hostname values

internal/store/store.go

@@ -1,12 +1,18 @@
 package store
 
 import (
+	"errors"
 	"sync"
 	"time"
 
 	"nerd-monitor/internal/stats"
 )
 
+var (
+	// ErrAgentNotFound is returned when an agent is not found.
+	ErrAgentNotFound = errors.New("agent not found")
+)
+
 // AgentStats represents the latest stats for an agent.
 type AgentStats struct {
 	ID        string
@@ -76,3 +82,17 @@ func (s *Store) DeleteAgent(id string) {
 
 	delete(s.agents, id)
 }
+
+// UpdateHostname updates the hostname for an agent.
+func (s *Store) UpdateHostname(id string, hostname string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	agent, exists := s.agents[id]
+	if !exists {
+		return ErrAgentNotFound
+	}
+
+	agent.Hostname = hostname
+	return nil
+}