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/ui/handlers.go

@@ -133,6 +133,30 @@ func (h *Handler) DeleteAgent(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/", http.StatusSeeOther)
 }
 
+// UpdateAgentHostname handles hostname updates for agents.
+func (h *Handler) UpdateAgentHostname(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	agentID := r.PathValue("id")
+	hostname := r.FormValue("hostname")
+
+	if hostname == "" {
+		http.Error(w, "Hostname cannot be empty", http.StatusBadRequest)
+		return
+	}
+
+	err := h.store.UpdateHostname(agentID, hostname)
+	if err != nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	http.Redirect(w, r, "/agents/"+agentID, http.StatusSeeOther)
+}
+
 // getStaleAgents returns agents that haven't reported in 6 months.
 func (h *Handler) getStaleAgents() []*store.AgentStats {
 	const staleThreshold = 6 * 30 * 24 * time.Hour