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/components/EnvironmentSelector.module.css

@@ -1,24 +1,4 @@
+/* Layout wrapper — DS Select handles its own appearance */
 .select {
-  appearance: none;
-  background: transparent;
-  border: none;
-  padding: 0 14px 0 0;
-  margin: 0;
-  font: inherit;
-  color: inherit;
-  text-transform: inherit;
-  letter-spacing: inherit;
-  cursor: pointer;
-  outline: none;
-  line-height: 1;
-  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='8' height='5' viewBox='0 0 8 5'%3E%3Cpath d='M1 1l3 3 3-3' stroke='currentColor' fill='none' stroke-width='1.5' stroke-linecap='round' stroke-linejoin='round'/%3E%3C/svg%3E");
-  background-repeat: no-repeat;
-  background-position: right 0 center;
-  background-size: 8px 5px;
-}
-
-.select:focus-visible {
-  outline: 1px solid var(--amber);
-  outline-offset: 2px;
-  border-radius: 2px;
+  min-width: 100px;
 }