feat: interactive ephemeral terminal per transcript session

Replace one-shot HTTP POST sendPrompt with a persistent ephemeral
terminal per session. Terminal auto-starts on session select, stays
running in background when modal is closed, and gets killed on
session switch or page unload.

- Add sendInput() to useEphemeralTerminal (text + Enter as separate WS messages)
- useTranscriptDebug owns terminal lifecycle (create/dispose on select/switch)
- ResumeTerminalButton receives shared terminal prop, only toggles modal
- UserInput shows "Starting terminal..." when not ready
- Add "New Session" button that starts a fresh agent session
- beforeunload sends sendBeacon to kill terminal on page close

frontend/src/components/FloatingTranscriptDebug.vue

@@ -30,9 +30,12 @@ const {
   error,
   isRealtime,
   processing,
+  ephemeral,
+  terminalReady,
   init,
   switchAgent,
   selectSession,
+  createNewSession,
   disconnectRealtime,
   sendPrompt
 } = useTranscriptDebug()
@@ -335,6 +338,10 @@ function handleSend(message: string) {
   sendPrompt(message)
 }
 
+function handleCreateSession() {
+  createNewSession()
+}
+
 // ============================================================================
 // WATCHERS
 // ============================================================================
@@ -485,6 +492,8 @@ onBeforeUnmount(() => {
               v-if="conversation"
               :conversation="conversation"
               :processing="processing"
+              :terminal-ready="terminalReady"
+              :terminal="ephemeral"
               :show-selector="showSelector"
               :agents="agents"
               :selected-agent="selectedAgent"
@@ -494,6 +503,7 @@ onBeforeUnmount(() => {
               @send="handleSend"
               @switch-agent="handleAgentSwitch"
               @select-session="handleSessionSelect"
+              @create-session="handleCreateSession"
             />
             <div v-else class="empty-state">
               <svg width="32" height="32" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5">