fix(terminal): prevent xterm.js crash, websocket disconnect cascade, stale sessions

Three related bugs fixed:

1. Frontend xterm.js crash: TerminalPage rendered ALL sessions with display:none
   for inactive ones. xterm.js crashes when initialized in a hidden container
   (Viewport can't read dimensions). Fix: only render the active session's
   TerminalComponent using conditional rendering.

2. Backend websocket disconnect cascade: When client disconnected (due to #1),
   the server tried to send 'connected' status on dead socket, caught the
   WebSocketDisconnect in a generic except block, then tried to close() again
   causing RuntimeError. Fix: catch WebSocketDisconnect specifically and suppress
   close() errors.

3. Stale DB sessions: After server restart, DB still had old terminal session
   rows but no in-memory sessions. list_terminal_sessions returned these ghosts,
   causing the frontend to render dead tabs. Fix: skip DB-only sessions that
   have no live in-memory counterpart.

Quality gates: pytest (15/15 passed), tsc clean, vitest (7/7 passed)

apps/web/src/pages/terminal.tsx

@@ -229,23 +229,22 @@ export const TerminalPage: React.FC = () => {
 				</div>
 				<div className="terminal-page-content">
 					{error && <div className="terminal-error-banner">{error}</div>}
-					{sessions.map((session) => (
-						<div
-							key={session.id}
-							className={`terminal-instance ${session.id === activeSessionId ? "active" : ""}`}
-							style={{
-								display: session.id === activeSessionId ? "flex" : "none",
-							}}
-						>
-							<TerminalComponent
-								ref={terminalRefs.current[session.id]}
-								instanceId={instanceId}
-								sessionId={session.id}
-								onClose={() => handleClose(session.id)}
-								isMobile={true}
-							/>
-						</div>
-					))}
+					{sessions
+						.filter((session) => session.id === activeSessionId)
+						.map((session) => (
+							<div
+								key={session.id}
+								className="terminal-instance active"
+							>
+								<TerminalComponent
+										ref={terminalRefs.current[session.id]}
+										instanceId={instanceId}
+										sessionId={session.id}
+										onClose={() => handleClose(session.id)}
+										isMobile={true}
+								/>
+							</div>
+						))}
 					{sessions.length === 0 && !loading && (
 						<div className="terminal-empty-state">
 							<p>No terminal sessions. Press Alt+Shift+N to create one.</p>
@@ -289,23 +288,22 @@ export const TerminalPage: React.FC = () => {
 			/>
 			<div className="terminal-page-content">
 				{error && <div className="terminal-error-banner">{error}</div>}
-				{sessions.map((session) => (
-					<div
-						key={session.id}
-						className={`terminal-instance ${session.id === activeSessionId ? "active" : ""}`}
-						style={{
-							display: session.id === activeSessionId ? "flex" : "none",
-						}}
-					>
-						<TerminalComponent
-							ref={terminalRefs.current[session.id]}
-							instanceId={instanceId}
-							sessionId={session.id}
-							onClose={() => handleClose(session.id)}
-							isMobile={false}
-						/>
-					</div>
-				))}
+				{sessions
+					.filter((session) => session.id === activeSessionId)
+					.map((session) => (
+						<div
+							key={session.id}
+							className="terminal-instance active"
+						>
+							<TerminalComponent
+								ref={terminalRefs.current[session.id]}
+								instanceId={instanceId}
+								sessionId={session.id}
+								onClose={() => handleClose(session.id)}
+								isMobile={false}
+							/>
+						</div>
+					))}
 				{sessions.length === 0 && !loading && (
 					<div className="terminal-empty-state">
 						<p>No terminal sessions. Press Alt+Shift+N to create one.</p>