style(host-detail): polish buttons, back link and port list

- Save changes / Add port: add btn-primary class for consistent blue accent
- Back button: stacked above page title, styled as a small bordered button
- Port list: remove row background, replace full border with bottom separator only

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

style/rust-ipam.css

@@ -1035,23 +1035,33 @@ td.col-actions {
   filter: brightness(0.9);
 }
 
-/* Left cluster inside page header (back link + title) */
+/* Left cluster inside page header: back button stacked above title */
 .page-header__left {
   display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: var(--size-xs);
+}
+
+.back-btn {
+  display: inline-flex;
   align-items: center;
-  gap: var(--size-md);
-}
-
-.back-link {
-  font-size: var(--font-sm);
+  gap: 4px;
+  font-size: var(--font-xs);
+  font-weight: 500;
   color: var(--text-secondary);
+  background: var(--bg-surface2);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  padding: 3px 10px;
   text-decoration: none;
-  white-space: nowrap;
+  cursor: pointer;
+  transition: background var(--transition-fast), color var(--transition-fast);
 }
 
-.back-link:hover {
+.back-btn:hover {
+  background: var(--bg-hover);
   color: var(--text);
-  text-decoration: underline;
 }
 
 /* Delete button inside hosts table */
@@ -1131,18 +1141,16 @@ td.col-actions {
 .port-list {
   display: flex;
   flex-direction: column;
-  gap: var(--size-xs);
   margin-bottom: var(--size-md);
+  border-top: 1px solid var(--border);
 }
 
 .port-row {
   display: flex;
   align-items: center;
   gap: var(--size-md);
-  padding: var(--size-sm) var(--size-sm);
-  border-radius: var(--radius-sm);
-  background: var(--bg);
-  border: 1px solid var(--border);
+  padding: var(--size-sm) var(--size-xs);
+  border-bottom: 1px solid var(--border);
 }
 
 .port-row__number {