fix: render notification dropdown via portal for true always-on-top

The notification dropdown was trapped inside .shell-header's stacking
context (created by backdrop-filter). Even with z-index: 9999, it
remained below any element with a higher root-level z-index such as
modal overlays (1000), dialog overlays (1000), and fullscreen
terminals (1000).

- Render the dropdown via ReactDOM.createPortal into document.body
  so it escapes all parent stacking contexts.
- Dynamically measure the bell button's bounding rect to position
  the dropdown correctly with position: fixed.
- Update click-outside handler to also ignore clicks on the bell
  button itself.
- Add window resize listener to keep dropdown aligned.
- Change .notification-dropdown from position: absolute to fixed.

Quality gates: tsc --noEmit (pass), build (pass)

apps/web/src/components/features/notification/notification-center.tsx

@@ -1,4 +1,5 @@
-import { useEffect, useRef } from "react";
+import { useEffect, useRef, useState, useCallback } from "react";
+import { createPortal } from "react-dom";
 import { useNotifications } from "../../../hooks/use-notifications";
 import { NotificationItem } from "./notification-item";
 import { Icon } from "../../icon";
@@ -22,15 +23,30 @@ export function NotificationCenter({
 		setIsDropdownOpen,
 	} = useNotifications();
 
+	const bellRef = useRef<HTMLButtonElement>(null);
 	const dropdownRef = useRef<HTMLDivElement>(null);
+	const [dropdownStyle, setDropdownStyle] = useState<React.CSSProperties>({});
+
+	const updatePosition = useCallback(() => {
+		if (!bellRef.current) return;
+		const rect = bellRef.current.getBoundingClientRect();
+		setDropdownStyle({
+			top: rect.bottom + 6,
+			right: window.innerWidth - rect.right,
+		});
+	}, []);
 
 	useEffect(() => {
 		if (!isDropdownOpen) return;
 
+		updatePosition();
+
 		const handleMouseDown = (e: MouseEvent) => {
+			const target = e.target as Node;
 			if (
 				dropdownRef.current &&
-				!dropdownRef.current.contains(e.target as Node)
+				!dropdownRef.current.contains(target) &&
+				!bellRef.current?.contains(target)
 			) {
 				setIsDropdownOpen(false);
 			}
@@ -42,14 +58,20 @@ export function NotificationCenter({
 			}
 		};
 
+		const handleResize = () => {
+			updatePosition();
+		};
+
 		document.addEventListener("mousedown", handleMouseDown);
 		document.addEventListener("keydown", handleKeyDown);
+		window.addEventListener("resize", handleResize);
 
 		return () => {
 			document.removeEventListener("mousedown", handleMouseDown);
 			document.removeEventListener("keydown", handleKeyDown);
+			window.removeEventListener("resize", handleResize);
 		};
-	}, [isDropdownOpen, setIsDropdownOpen]);
+	}, [isDropdownOpen, setIsDropdownOpen, updatePosition]);
 
 	useEffect(() => {
 		if (isDropdownOpen) {
@@ -66,6 +88,7 @@ export function NotificationCenter({
 	return (
 		<div className="notification-center">
 			<button
+				ref={bellRef}
 				type="button"
 				className="notification-bell"
 				onClick={() => setIsDropdownOpen(!isDropdownOpen)}
@@ -79,56 +102,59 @@ export function NotificationCenter({
 				)}
 			</button>
 
-			{isDropdownOpen && (
-				<div
-					ref={dropdownRef}
-					role="dialog"
-					aria-label="Notifications"
-					className="notification-dropdown"
-				>
-					<div className="notification-dropdown-header">
-						<span>Notifications</span>
-					</div>
-
-					<ul className="notification-list">
-						{notifications.length === 0 ? (
-							<li className="notification-empty">No notifications</li>
-						) : (
-							notifications.map((n) => (
-								<NotificationItem
-									key={n.id}
-									notification={n}
-									onMarkRead={markRead}
-									onDismiss={dismiss}
-								/>
-							))
-						)}
-					</ul>
-
-					{notifications.length > 0 && (
-						<div className="notification-dropdown-footer">
-							<button
-								type="button"
-								className="notification-mark-all"
-								onClick={() => {
-									void markAllRead();
-								}}
-							>
-								Mark all as read
-							</button>
-							<button
-								type="button"
-								className="notification-clear-all"
-								onClick={() => {
-									void clearAll();
-								}}
-							>
-								Clear all
-							</button>
+			{isDropdownOpen &&
+				createPortal(
+					<div
+						ref={dropdownRef}
+						role="dialog"
+						aria-label="Notifications"
+						className="notification-dropdown"
+						style={dropdownStyle}
+					>
+						<div className="notification-dropdown-header">
+							<span>Notifications</span>
 						</div>
-					)}
-				</div>
-			)}
+
+						<ul className="notification-list">
+							{notifications.length === 0 ? (
+								<li className="notification-empty">No notifications</li>
+							) : (
+								notifications.map((n) => (
+									<NotificationItem
+										key={n.id}
+										notification={n}
+										onMarkRead={markRead}
+										onDismiss={dismiss}
+									/>
+								))
+							)}
+						</ul>
+
+						{notifications.length > 0 && (
+							<div className="notification-dropdown-footer">
+								<button
+									type="button"
+									className="notification-mark-all"
+									onClick={() => {
+										void markAllRead();
+									}}
+								>
+									Mark all as read
+								</button>
+								<button
+									type="button"
+									className="notification-clear-all"
+									onClick={() => {
+										void clearAll();
+									}}
+								>
+									Clear all
+								</button>
+							</div>
+						)}
+					</div>,
+					document.body,
+				)}
 		</div>
 	);
 }