Add rack height field to create form

The rack creation form now includes a Height (U) input defaulting to 42.
The handler reads it instead of hardcoding. Previously every rack was
silently locked at 42U until you went to the rack page to edit it.

internal/handlers/overview.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"net/http"
+	"strconv"
 
 	"lostcavewireplanner/internal/models"
 )
@@ -47,6 +48,11 @@ func (h *Handlers) RackCreate(w http.ResponseWriter, r *http.Request) {
 	rackType := r.FormValue("rack_type")
 	depth := r.FormValue("depth")
 	heightUnits := 42
+	if huStr := r.FormValue("height_units"); huStr != "" {
+		if n, err := strconv.Atoi(huStr); err == nil && n > 0 {
+			heightUnits = n
+		}
+	}
 
 	if name == "" {
 		h.renderError(w, "overview.html", "Name is required")

templates/overview.html

@@ -21,6 +21,7 @@
     <label>Name <input name="name" required></label>
     <label>Type <select name="rack_type"><option value="network">Network</option><option value="server">Server</option></select></label>
     <label>Depth <select name="depth"><option value="shallow">Shallow</option><option value="deep">Deep</option></select></label>
+    <label>Height (U) <input type="number" name="height_units" value="42" min="1" style="width:6em"></label>
     <button type="submit">Create Rack</button>
 </form>
 </details>