feat: notification center toast coordination (PR-4)

- EventToastBridge checks notification_toast_level and notification_mute_categories
- toast-rules.ts: event-to-category/severity mapping functions
- Settings page: notification preferences section (toast level dropdown, mute checkboxes)
- Settings API types extended with notification preference fields
- 17 frontend tests (toast-rules + bridge)
- Preference hierarchy: mute categories → toast level → show/hide

Quality gates: vitest 17 passed, tsc clean, eslint clean

apps/web/src/components/event-toast-bridge.tsx

@@ -1,19 +1,77 @@
-import { useEffect, useRef } from "react";
+import { useEffect, useRef, useState } from "react";
 import { useEventContext } from "../state/events";
-import { handleEventToast } from "./toast-rules";
+import {
+	handleEventToast,
+	mapEventToCategory,
+	mapEventToSeverity,
+} from "./toast-rules";
+import { getUserConfig } from "../api/settings";
+import type { UserConfig } from "../api/settings";
+
+interface ToastConfig {
+	notification_toast_level: string;
+	notification_mute_categories: string[];
+}
 
 export function EventToastBridge(): JSX.Element | null {
 	const { events } = useEventContext();
 	const processedRef = useRef<Set<string>>(new Set());
+	const [config, setConfig] = useState<ToastConfig | null>(null);
 
 	useEffect(() => {
+		getUserConfig()
+			.then((c) => {
+				setConfig({
+					notification_toast_level: c.notification_toast_level ?? "all",
+					notification_mute_categories: c.notification_mute_categories ?? [],
+				});
+			})
+			.catch(() => {
+				setConfig({
+					notification_toast_level: "all",
+					notification_mute_categories: [],
+				});
+			});
+
+		const handler = (e: Event) => {
+			const detail = (e as CustomEvent<Partial<UserConfig>>).detail;
+			if (detail) {
+				setConfig((prev) => ({
+					notification_toast_level:
+						detail.notification_toast_level ??
+						prev?.notification_toast_level ??
+						"all",
+					notification_mute_categories:
+						detail.notification_mute_categories ??
+						prev?.notification_mute_categories ??
+						[],
+				}));
+			}
+		};
+
+		window.addEventListener("userconfig:updated", handler);
+		return () => window.removeEventListener("userconfig:updated", handler);
+	}, []);
+
+	useEffect(() => {
+		if (!config) return;
+
 		for (const event of events) {
 			const key = `${event.correlation_id}:${event.timestamp}`;
 			if (processedRef.current.has(key)) continue;
 			processedRef.current.add(key);
+
+			const category = mapEventToCategory(event);
+			const severity = mapEventToSeverity(event);
+
+			if (config.notification_toast_level === "none") continue;
+			if (config.notification_toast_level === "errors" && severity !== "error")
+				continue;
+			if (config.notification_mute_categories.includes(category)) continue;
+
 			handleEventToast(event);
 		}
-	}, [events]);
+	}, [events, config]);
 
 	return null;
 }