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.

server/services/terminal.ts

@@ -1,7 +1,33 @@
 import { spawn, type IPty } from '@skitee3000/bun-pty'
+import { existsSync } from 'fs'
+import { join } from 'path'
+import { homedir } from 'os'
 import { PORT_TERMINAL, WORKING_DIR, SHELL, SHELL_ARGS, DEFAULT_SESSION_ID, MAX_BUFFER_LINES, MAX_TERMINALS } from '../config'
 import { sessionState, type SessionStatePatch } from './session-state'
 
+// Agent transcript directories (mirrored from transcript-debug.ts)
+const AGENT_TRANSCRIPT_DIRS: Record<string, string> = {
+  ejecutor: join(WORKING_DIR, '.claude-ejecutor', 'projects'),
+  nucleo000: join(WORKING_DIR, '.claude-nucleo000', 'projects'),
+  claude: join(homedir(), '.claude', 'projects')
+}
+
+const PROJECT_HASH = 'C--Users-jodar-agent-ui'
+
+function getTranscriptProjectDir(agent: string): string | null {
+  const baseDir = AGENT_TRANSCRIPT_DIRS[agent]
+  if (!baseDir || !existsSync(baseDir)) return null
+  const exact = join(baseDir, PROJECT_HASH)
+  if (existsSync(exact)) return exact
+  return null
+}
+
+function transcriptSessionExists(agent: string, sessionId: string): boolean {
+  const projectDir = getTranscriptProjectDir(agent)
+  if (!projectDir) return false
+  return existsSync(join(projectDir, `${sessionId}.jsonl`))
+}
+
 interface TerminalSession {
   id: string
   pty: IPty
@@ -400,6 +426,18 @@ export function startTerminalServer() {
             )
           }
 
+          // Validate transcript session exists before resuming
+          const tsId = body.transcriptSessionId
+          if (tsId && tsId !== '__new__' && body.agent) {
+            if (!transcriptSessionExists(body.agent, tsId)) {
+              console.warn(`[Terminal] Transcript session not found: ${tsId} (agent: ${body.agent})`)
+              return Response.json(
+                { error: `Transcript session "${tsId}" not found. It may have been deleted.` },
+                { status: 404, headers: corsHeaders }
+              )
+            }
+          }
+
           // Generate ephemeralSessionId server-side
           const ephemeralSessionId = `pty-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`