fix: validate transcript sessions before resume and fix FAB race condition

Server now checks that transcript .jsonl files exist before creating
terminals, preventing dead sessions from --resume errors. Frontend
shows error banner in modal when resume fails. Fixed race condition
where init() would overwrite FAB terminal selection after page refresh
by guarding with pendingSwitchTarget flag.

frontend/src/components/transcript-debug/NewSessionModal.vue

@@ -6,6 +6,7 @@ const props = defineProps<{
   visible: boolean
   agents: { id: AgentName; label: string }[]
   currentAgent: AgentName
+  error?: string | null
 }>()
 
 const emit = defineEmits<{
@@ -34,7 +35,8 @@ const hasAnySessions = computed(() =>
 // Reset state when modal opens
 watch(() => props.visible, async (open) => {
   if (open) {
-    activeTab.value = 'new'
+    // If reopening after a resume error, show the resume tab
+    activeTab.value = props.error ? 'resume' : 'new'
     selectedAgent.value = props.currentAgent
     resumeFilter.value = 'all'
     initialPrompt.value = ''
@@ -129,6 +131,16 @@ function handleResume(sessionId: string, agent: AgentName) {
             </button>
           </div>
 
+          <!-- Error banner -->
+          <div v-if="error" class="nsm-error">
+            <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+              <circle cx="12" cy="12" r="10"/>
+              <line x1="12" y1="8" x2="12" y2="12"/>
+              <line x1="12" y1="16" x2="12.01" y2="16"/>
+            </svg>
+            {{ error }}
+          </div>
+
           <!-- Body -->
           <div class="nsm-body">
             <!-- Tab: New session -->
@@ -333,6 +345,24 @@ function handleResume(sessionId: string, agent: AgentName) {
   border-bottom-color: #6366f1;
 }
 
+/* Error banner */
+.nsm-error {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem 1rem;
+  background: rgba(239, 68, 68, 0.12);
+  border-bottom: 1px solid rgba(239, 68, 68, 0.3);
+  color: #f87171;
+  font-size: 12px;
+  flex-shrink: 0;
+}
+
+.nsm-error svg {
+  flex-shrink: 0;
+  color: #ef4444;
+}
+
 /* Body */
 .nsm-body {
   flex: 1;