feat(terminal-dashboard): improve UX for pane/session management

- Add confirmation dialog when closing pane with active session
- Add explicit "Close Session" button to terminate backend PTY
- Handle "session not found" scenario with user-friendly message
- Add i18n keys for new UI elements (en/zh)

ccw/frontend/src/components/terminal-dashboard/TerminalPane.tsx

@@ -6,7 +6,7 @@
 // Renders within the TerminalGrid recursive layout.
 // File preview is triggered from right sidebar FileSidebarPanel.
 
-import { useCallback, useMemo, useState } from 'react';
+import { useCallback, useEffect, useMemo, useState } from 'react';
 import { useIntl } from 'react-intl';
 import {
   SplitSquareHorizontal,
@@ -23,6 +23,7 @@ import {
   FileText,
   ArrowLeft,
   LogOut,
+  WifiOff,
 } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import { TerminalInstance } from './TerminalInstance';
@@ -64,6 +65,7 @@ const statusDotStyles: Record<TerminalStatus, string> = {
   error: 'bg-red-500',
   paused: 'bg-yellow-500',
   resuming: 'bg-blue-400 animate-pulse',
+  locked: 'bg-amber-500 animate-pulse',
 };
 
 // ========== Props ==========
@@ -129,6 +131,20 @@ export function TerminalPane({ paneId }: TerminalPaneProps) {
   const status: TerminalStatus = meta?.status ?? 'idle';
   const alertCount = meta?.alertCount ?? 0;
 
+  // Check if session exists - handles case where pane references a sessionId
+  // that no longer exists in cliSessionStore (e.g., after page refresh if backend restarted)
+  const isSessionNotFound = sessionId && !sessions[sessionId];
+
+  // Clear invalid sessionId after showing the message
+  useEffect(() => {
+    if (isSessionNotFound) {
+      const timer = setTimeout(() => {
+        assignSession(paneId, null);
+      }, 3000); // Clear after 3 seconds to let user see the message
+      return () => clearTimeout(timer);
+    }
+  }, [isSessionNotFound, paneId, assignSession]);
+
   // Build session options for dropdown
   // Use sessions from cliSessionStore directly (all sessions, not just grouped ones)
   const sessionOptions = useMemo(() => {
@@ -261,7 +277,9 @@ export function TerminalPane({ paneId }: TerminalPaneProps) {
           ) : (
             // Terminal mode header
             <>
-              {sessionId && (
+              {isSessionNotFound ? (
+                <WifiOff className="w-3.5 h-3.5 text-yellow-500 shrink-0" />
+              ) : sessionId && (
                 <span
                   className={cn('w-2 h-2 rounded-full shrink-0', statusDotStyles[status])}
                 />
@@ -314,7 +332,7 @@ export function TerminalPane({ paneId }: TerminalPaneProps) {
           >
             <SplitSquareVertical className="w-3.5 h-3.5" />
           </button>
-          {!isFileMode && sessionId && (
+          {!isFileMode && sessionId && !isSessionNotFound && (
             <>
               {/* Restart button */}
               <button
@@ -417,6 +435,19 @@ export function TerminalPane({ paneId }: TerminalPaneProps) {
             className="h-full"
           />
         </div>
+      ) : isSessionNotFound ? (
+        // Session not found state - pane references a sessionId that no longer exists
+        <div className="flex-1 flex items-center justify-center text-muted-foreground">
+          <div className="text-center px-4">
+            <WifiOff className="h-6 w-6 mx-auto mb-1.5 text-yellow-500" />
+            <p className="text-sm font-medium text-foreground">
+              {formatMessage({ id: 'terminalDashboard.pane.sessionNotFound' })}
+            </p>
+            <p className="text-xs mt-1 opacity-70 max-w-[200px]">
+              {formatMessage({ id: 'terminalDashboard.pane.sessionNotFoundHint' })}
+            </p>
+          </div>
+        </div>
       ) : sessionId ? (
         // Terminal mode with session
         <div className="flex-1 min-h-0">

ccw/frontend/src/locales/en/terminal-dashboard.json

@@ -143,6 +143,8 @@
   "pane": {
     "selectSession": "Select a session",
     "selectSessionHint": "Choose a terminal session from the dropdown",
+    "sessionNotFound": "Session no longer exists",
+    "sessionNotFoundHint": "The session may have been closed or the server restarted. Please select another session.",
     "splitHorizontal": "Split Right",
     "splitVertical": "Split Down",
     "clearTerminal": "Clear Terminal",

ccw/frontend/src/locales/zh/terminal-dashboard.json

@@ -143,6 +143,8 @@
   "pane": {
     "selectSession": "选择会话",
     "selectSessionHint": "从下拉菜单中选择终端会话",
+    "sessionNotFound": "会话已不存在",
+    "sessionNotFoundHint": "该会话可能已关闭或服务器已重启，请选择其他会话。",
     "splitHorizontal": "向右分割",
     "splitVertical": "向下分割",
     "clearTerminal": "清屏",

ccw/frontend/src/stores/sessionManagerStore.ts

@@ -361,6 +361,56 @@ export const useSessionManagerStore = create<SessionManagerStore>()(
           throw error;
         }
       },
+
+      // ========== Session Lock Actions ==========
+
+      lockSession: (sessionId: string, reason: string, executionId?: string) => {
+        set(
+          (state) => {
+            const existing = state.terminalMetas[sessionId];
+            if (!existing) return state;
+            return {
+              terminalMetas: {
+                ...state.terminalMetas,
+                [sessionId]: {
+                  ...existing,
+                  status: 'locked' as TerminalStatus,
+                  isLocked: true,
+                  lockReason: reason,
+                  lockedByExecutionId: executionId,
+                  lockedAt: new Date().toISOString(),
+                },
+              },
+            };
+          },
+          false,
+          'lockSession'
+        );
+      },
+
+      unlockSession: (sessionId: string) => {
+        set(
+          (state) => {
+            const existing = state.terminalMetas[sessionId];
+            if (!existing) return state;
+            return {
+              terminalMetas: {
+                ...state.terminalMetas,
+                [sessionId]: {
+                  ...existing,
+                  status: 'active' as TerminalStatus,
+                  isLocked: false,
+                  lockReason: undefined,
+                  lockedByExecutionId: undefined,
+                  lockedAt: undefined,
+                },
+              },
+            };
+          },
+          false,
+          'unlockSession'
+        );
+      },
     }),
     { name: 'SessionManagerStore' }
   )

ccw/frontend/src/types/terminal-dashboard.ts

@@ -18,7 +18,7 @@ export interface SessionLayout {
 }
 
 /** Terminal status indicator */
-export type TerminalStatus = 'active' | 'idle' | 'error' | 'paused' | 'resuming';
+export type TerminalStatus = 'active' | 'idle' | 'error' | 'paused' | 'resuming' | 'locked';
 
 /** Metadata for a terminal instance in the dashboard */
 export interface TerminalMeta {
@@ -28,6 +28,14 @@ export interface TerminalMeta {
   status: TerminalStatus;
   /** Number of unread alerts (errors, warnings) */
   alertCount: number;
+  /** Whether the session is locked (executing a workflow) */
+  isLocked?: boolean;
+  /** Reason for the lock (e.g., workflow name) */
+  lockReason?: string;
+  /** Execution ID that locked this session */
+  lockedByExecutionId?: string;
+  /** Timestamp when the session was locked */
+  lockedAt?: string;
 }
 
 /** Group of terminal sessions */
@@ -91,6 +99,10 @@ export interface SessionManagerActions {
   restartSession: (terminalId: string) => Promise<void>;
   /** Close and terminate a terminal session permanently */
   closeSession: (terminalId: string) => Promise<void>;
+  /** Lock a session to prevent user input during workflow execution */
+  lockSession: (sessionId: string, reason: string, executionId?: string) => void;
+  /** Unlock a session after workflow execution completes */
+  unlockSession: (sessionId: string) => void;
 }
 
 export type SessionManagerStore = SessionManagerState & SessionManagerActions;