feat(hosts): replace inline add form with modal dialog

The add-host form is now opened via a "+ Add host" button in the page
header. The modal closes on Cancel, backdrop click, × button, or
automatically after a successful creation.

Adds modal CSS with backdrop blur and entry animation, .btn-primary /
.btn-secondary shared button styles, and a .page-header flex layout
reusable across list pages.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

style/rust-ipam.css

@@ -859,35 +859,142 @@ td.col-actions {
 }
 
 /* ============================================================
-   HOSTS PAGE
+   MODAL
    ============================================================ */
 
-.hosts-page h1 {
-  margin-bottom: var(--size-lg);
+.modal-backdrop {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.45);
+  backdrop-filter: blur(3px);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 200;
+  animation: backdrop-in var(--transition-base) both;
 }
 
-.hosts-page .add-form {
+@keyframes backdrop-in {
+  from { opacity: 0; }
+  to   { opacity: 1; }
+}
+
+.modal {
   background: var(--bg-surface);
   border: 1px solid var(--border);
   border-radius: var(--radius-lg);
-  box-shadow: var(--shadow-sm);
-  padding: var(--size-lg);
-  margin-bottom: var(--size-md);
+  box-shadow: var(--shadow-lg);
+  padding: var(--size-lg) var(--size-xl);
+  width: 90%;
+  max-width: 440px;
+  animation: modal-in var(--transition-base) both;
 }
 
-.hosts-page .add-form h2 {
-  margin-bottom: var(--size-md);
+@keyframes modal-in {
+  from { opacity: 0; transform: translateY(-12px) scale(0.97); }
+  to   { opacity: 1; transform: translateY(0)     scale(1);    }
 }
 
-.add-form__fields {
-  display: grid;
-  grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
+.modal__header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: var(--size-lg);
+}
+
+.modal__header h2 {
+  margin: 0;
+  font-size: var(--font-lg);
+}
+
+.modal__close {
+  background: none;
+  border: none;
+  font-size: 1.4rem;
+  line-height: 1;
+  cursor: pointer;
+  color: var(--text-secondary);
+  padding: 2px 6px;
+  border-radius: var(--radius-sm);
+  transition: color var(--transition-fast), background var(--transition-fast);
+}
+
+.modal__close:hover {
+  color: var(--text);
+  background: var(--bg-hover);
+}
+
+/* Form fields inside modal — single column stack */
+.modal .add-form__fields {
+  display: flex;
+  flex-direction: column;
   gap: var(--size-md);
-  align-items: end;
 }
 
-.add-form__fields button[type="submit"] {
-  align-self: end;
+.modal .add-form__fields label {
+  display: flex;
+  flex-direction: column;
+  gap: var(--size-xs);
+  font-size: var(--font-sm);
+  font-weight: 500;
+  color: var(--text-secondary);
+}
+
+/* Cancel / Submit button row */
+.modal__actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: var(--size-sm);
+  margin-top: var(--size-lg);
+}
+
+.btn-secondary {
+  background: var(--bg-hover);
+  color: var(--text);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  padding: 7px var(--size-md);
+  font-size: var(--font-sm);
+  font-weight: 500;
+  cursor: pointer;
+  transition: background var(--transition-fast);
+}
+
+.btn-secondary:hover {
+  background: var(--bg-surface2);
+}
+
+/* ============================================================
+   HOSTS PAGE
+   ============================================================ */
+
+/* Header row: title on the left, "Add host" button on the right */
+.page-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: var(--size-lg);
+}
+
+.page-header h1 {
+  margin: 0;
+}
+
+.btn-primary {
+  background: var(--accent);
+  color: var(--text-on-accent);
+  border: none;
+  border-radius: var(--radius-sm);
+  padding: 8px var(--size-md);
+  font-size: var(--font-sm);
+  font-weight: 500;
+  cursor: pointer;
+  transition: background var(--transition-fast);
+  white-space: nowrap;
+}
+
+.btn-primary:hover {
+  background: var(--accent-hover);
 }
 
 /* Delete button inside hosts table */