feat: make notification center mobile friendly

- Use useMobileViewport to detect mobile and position dropdown
  centered with left/right margins instead of right-aligned, which
  caused overflow on small screens.
- Add a semi-transparent backdrop overlay on mobile so tapping
  outside the dropdown naturally closes it.
- Update mobile CSS: notification-dropdown fills screen width
  with 0.75rem margins, max-height capped at 70vh for reachability.
- Remove the 360px max-width cap on mobile so the dropdown uses
  available screen space properly.

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

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

@@ -1,6 +1,7 @@
 import { useEffect, useRef, useState, useCallback } from "react";
 import { createPortal } from "react-dom";
 import { useNotifications } from "../../../hooks/use-notifications";
+import { useMobileViewport } from "../../../hooks/use-mobile-viewport";
 import { NotificationItem } from "./notification-item";
 import { Icon } from "../../icon";
 
@@ -23,6 +24,7 @@ export function NotificationCenter({
 		setIsDropdownOpen,
 	} = useNotifications();
 
+	const isMobile = useMobileViewport();
 	const bellRef = useRef<HTMLButtonElement>(null);
 	const dropdownRef = useRef<HTMLDivElement>(null);
 	const [dropdownStyle, setDropdownStyle] = useState<React.CSSProperties>({});
@@ -30,11 +32,19 @@ export function NotificationCenter({
 	const updatePosition = useCallback(() => {
 		if (!bellRef.current) return;
 		const rect = bellRef.current.getBoundingClientRect();
-		setDropdownStyle({
-			top: rect.bottom + 6,
-			right: window.innerWidth - rect.right,
-		});
-	}, []);
+		if (isMobile) {
+			setDropdownStyle({
+				top: rect.bottom + 6,
+				left: "1rem",
+				right: "1rem",
+			});
+		} else {
+			setDropdownStyle({
+				top: rect.bottom + 6,
+				right: window.innerWidth - rect.right,
+			});
+		}
+	}, [isMobile]);
 
 	useEffect(() => {
 		if (!isDropdownOpen) return;
@@ -104,55 +114,64 @@ export function NotificationCenter({
 
 			{isDropdownOpen &&
 				createPortal(
-					<div
-						ref={dropdownRef}
-						role="dialog"
-						aria-label="Notifications"
-						className="notification-dropdown"
-						style={dropdownStyle}
-					>
-						<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>
-							</div>
+					<>
+						{isMobile && (
+							<div
+								className="notification-dropdown-backdrop"
+								onClick={() => setIsDropdownOpen(false)}
+								aria-hidden="true"
+							/>
 						)}
-					</div>,
+						<div
+							ref={dropdownRef}
+							role="dialog"
+							aria-label="Notifications"
+							className="notification-dropdown"
+							style={dropdownStyle}
+						>
+							<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>
+								</div>
+							)}
+						</div>
+					</>,
 					document.body,
 				)}
 		</div>

apps/web/src/styles.css

@@ -4838,9 +4838,20 @@ a:active,
 }
 
 @media (max-width: 767px) {
+	.notification-dropdown-backdrop {
+		position: fixed;
+		inset: 0;
+		background: rgba(0, 0, 0, 0.35);
+		z-index: 9998;
+	}
+
 	.notification-dropdown {
-		width: calc(100vw - 2rem);
-		max-width: 360px;
+		width: auto;
+		left: 0.75rem;
+		right: 0.75rem;
+		max-width: none;
+		border-radius: 12px;
+		max-height: 70vh;
 	}
 }