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/AgentHealth/AgentHealth.module.css

@@ -58,102 +58,12 @@
   color: var(--text-muted);
 }
 
-.configSelect {
-  padding: 5px 10px;
-  border: 1px solid var(--border-subtle);
-  border-radius: var(--radius-sm);
-  background: var(--bg-body);
-  color: var(--text-primary);
-  font-size: 12px;
-  font-family: var(--font-mono);
-  outline: none;
-  cursor: pointer;
-}
-
-.configSelect:focus {
-  border-color: var(--amber);
-}
-
-.configSelect:disabled {
-  opacity: 0.5;
-  cursor: not-allowed;
-}
-
-.configToggle {
-  display: flex;
-  align-items: center;
-  gap: 6px;
-  font-size: 12px;
-  font-family: var(--font-mono);
-  color: var(--text-secondary);
-  cursor: pointer;
-}
-
-.configToggle input {
-  accent-color: var(--amber);
-  cursor: pointer;
-}
-
-.configEditBtn {
-  background: transparent;
-  border: none;
-  color: var(--text-faint);
-  opacity: 0.75;
-  cursor: pointer;
-  font-size: 14px;
-  padding: 4px 6px;
-  border-radius: var(--radius-sm);
-  line-height: 1;
-  display: inline-flex;
-  align-self: flex-end;
-}
-
-.configEditBtn:hover {
-  color: var(--text-primary);
-  opacity: 1;
-}
-
 .configActions {
   display: flex;
   gap: 8px;
   align-self: flex-end;
 }
 
-.configSaveBtn {
-  padding: 4px 12px;
-  border: none;
-  border-radius: var(--radius-sm);
-  background: var(--amber);
-  color: var(--bg-surface);
-  font-size: 12px;
-  font-weight: 600;
-  cursor: pointer;
-}
-
-.configSaveBtn:hover {
-  background: var(--amber-deep);
-}
-
-.configSaveBtn:disabled {
-  opacity: 0.5;
-  cursor: not-allowed;
-}
-
-.configCancelBtn {
-  padding: 4px 12px;
-  border: 1px solid var(--border-subtle);
-  border-radius: var(--radius-sm);
-  background: transparent;
-  color: var(--text-muted);
-  font-size: 12px;
-  cursor: pointer;
-}
-
-.configCancelBtn:hover {
-  color: var(--text-primary);
-  border-color: var(--text-faint);
-}
-
 /* Section header */
 .sectionTitle {
   font-size: 13px;