refactor: replace native HTML with design system components (Phase 5)

- EnvironmentSelector: bare <select> -> DS Select
- LogTab: raw <table> + <input> + <button> -> DS LogViewer + Input + Button
- AppsTab: 3 homegrown sub-tab bars -> DS Tabs, remove unused CSS
- AppConfigDetailPage: 4x <select> -> DS Select, 2x <input checkbox> ->
  DS Toggle, 7x <label> -> DS Label, 4x <button> -> DS Button
- AgentHealth: 4x <select> -> DS Select, 7x <button> -> DS Button

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/AppsTab/AppsTab.module.css

@@ -119,34 +119,6 @@
   gap: 8px;
 }
 
-/* Sub tabs */
-.subTabs {
-  display: flex;
-  border-bottom: 1px solid var(--border-subtle);
-  margin-bottom: 16px;
-}
-
-.subTab {
-  padding: 8px 16px;
-  font-size: 13px;
-  font-weight: 500;
-  color: var(--text-muted);
-  cursor: pointer;
-  border: none;
-  border-bottom: 2px solid transparent;
-  background: none;
-  font-family: var(--font-body);
-}
-
-.subTab:hover {
-  color: var(--text-primary);
-}
-
-.subTabActive {
-  color: var(--amber);
-  border-bottom-color: var(--amber);
-}
-
 /* Table */
 .table {
   width: 100%;