Add syntax highlighting in code blocks and OS push notifications

Syntax highlighting: shared markdown renderer with highlight.js
(atom-one-dark theme), 12 language grammars registered (JS, TS,
Python, Rust, Go, Bash, JSON, YAML, SQL, CSS, HTML, Markdown).
Applied to both article reader and editor preview.

OS notifications: Tauri notification plugin for mentions, DMs, and
zaps. Per-type toggles in Settings with custom toggle switches.
Fires on new unread mentions/DMs; requests OS permission on first
enable. Notification utility at src/lib/notifications.ts.

src/lib/notifications.ts

@@ -0,0 +1,67 @@
+import {
+  isPermissionGranted,
+  requestPermission,
+  sendNotification,
+} from "@tauri-apps/plugin-notification";
+
+const SETTINGS_KEY = "wrystr_notification_settings";
+
+interface NotificationSettings {
+  mentions: boolean;
+  dms: boolean;
+  zaps: boolean;
+}
+
+const defaults: NotificationSettings = { mentions: true, dms: true, zaps: true };
+
+export function getNotificationSettings(): NotificationSettings {
+  try {
+    const stored = localStorage.getItem(SETTINGS_KEY);
+    return stored ? { ...defaults, ...JSON.parse(stored) } : defaults;
+  } catch {
+    return defaults;
+  }
+}
+
+export function saveNotificationSettings(settings: NotificationSettings): void {
+  localStorage.setItem(SETTINGS_KEY, JSON.stringify(settings));
+}
+
+export async function ensurePermission(): Promise<boolean> {
+  let granted = await isPermissionGranted();
+  if (!granted) {
+    const result = await requestPermission();
+    granted = result === "granted";
+  }
+  return granted;
+}
+
+export async function notifyMention(authorName: string, preview: string): Promise<void> {
+  const settings = getNotificationSettings();
+  if (!settings.mentions) return;
+  if (!(await ensurePermission())) return;
+  sendNotification({
+    title: `${authorName} mentioned you`,
+    body: preview.slice(0, 120),
+  });
+}
+
+export async function notifyDM(authorName: string, preview: string): Promise<void> {
+  const settings = getNotificationSettings();
+  if (!settings.dms) return;
+  if (!(await ensurePermission())) return;
+  sendNotification({
+    title: `DM from ${authorName}`,
+    body: preview.slice(0, 120),
+  });
+}
+
+export async function notifyZap(senderName: string, amount: number): Promise<void> {
+  const settings = getNotificationSettings();
+  if (!settings.zaps) return;
+  if (!(await ensurePermission())) return;
+  sendNotification({
+    title: `${senderName} zapped you`,
+    body: `${amount.toLocaleString()} sats`,
+  });
+}