feat: add toast notification system with success toasts

Adds ToastProvider/ToastViewport for in-app notifications with dedupe,
auto-dismiss, and action links. Wires success toasts to issue create,
issue update, and comment mutations. Adds live event toasts for activity,
agent status, and run status changes via LiveUpdatesProvider.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/context/ToastContext.tsx

@@ -0,0 +1,167 @@
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useRef,
+  useState,
+  type ReactNode,
+} from "react";
+
+export type ToastTone = "info" | "success" | "warn" | "error";
+
+export interface ToastAction {
+  label: string;
+  href: string;
+}
+
+export interface ToastInput {
+  id?: string;
+  dedupeKey?: string;
+  title: string;
+  body?: string;
+  tone?: ToastTone;
+  ttlMs?: number;
+  action?: ToastAction;
+}
+
+export interface ToastItem {
+  id: string;
+  title: string;
+  body?: string;
+  tone: ToastTone;
+  ttlMs: number;
+  action?: ToastAction;
+  createdAt: number;
+}
+
+interface ToastContextValue {
+  toasts: ToastItem[];
+  pushToast: (input: ToastInput) => string | null;
+  dismissToast: (id: string) => void;
+  clearToasts: () => void;
+}
+
+const DEFAULT_TTL_MS = 5000;
+const MIN_TTL_MS = 1500;
+const MAX_TTL_MS = 15000;
+const MAX_TOASTS = 5;
+const DEDUPE_WINDOW_MS = 3500;
+const DEDUPE_MAX_AGE_MS = 20000;
+
+const ToastContext = createContext<ToastContextValue | null>(null);
+
+function normalizeTtl(value: number | undefined) {
+  const fallback = DEFAULT_TTL_MS;
+  if (typeof value !== "number" || !Number.isFinite(value)) return fallback;
+  return Math.max(MIN_TTL_MS, Math.min(MAX_TTL_MS, Math.floor(value)));
+}
+
+function generateToastId() {
+  return `toast_${Date.now()}_${Math.random().toString(36).slice(2, 8)}`;
+}
+
+export function ToastProvider({ children }: { children: ReactNode }) {
+  const [toasts, setToasts] = useState<ToastItem[]>([]);
+  const timersRef = useRef(new Map<string, number>());
+  const dedupeRef = useRef(new Map<string, number>());
+
+  const clearTimer = useCallback((id: string) => {
+    const handle = timersRef.current.get(id);
+    if (handle !== undefined) {
+      window.clearTimeout(handle);
+      timersRef.current.delete(id);
+    }
+  }, []);
+
+  const dismissToast = useCallback(
+    (id: string) => {
+      clearTimer(id);
+      setToasts((prev) => prev.filter((toast) => toast.id !== id));
+    },
+    [clearTimer],
+  );
+
+  const clearToasts = useCallback(() => {
+    for (const handle of timersRef.current.values()) {
+      window.clearTimeout(handle);
+    }
+    timersRef.current.clear();
+    setToasts([]);
+  }, []);
+
+  const pushToast = useCallback(
+    (input: ToastInput) => {
+      const now = Date.now();
+      const tone = input.tone ?? "info";
+      const ttlMs = normalizeTtl(input.ttlMs);
+      const dedupeKey =
+        input.dedupeKey ?? input.id ?? `${tone}|${input.title}|${input.body ?? ""}|${input.action?.href ?? ""}`;
+
+      for (const [key, ts] of dedupeRef.current.entries()) {
+        if (now - ts > DEDUPE_MAX_AGE_MS) {
+          dedupeRef.current.delete(key);
+        }
+      }
+
+      const lastSeen = dedupeRef.current.get(dedupeKey);
+      if (lastSeen && now - lastSeen < DEDUPE_WINDOW_MS) {
+        return null;
+      }
+      dedupeRef.current.set(dedupeKey, now);
+
+      const id = input.id ?? generateToastId();
+      clearTimer(id);
+
+      setToasts((prev) => {
+        const nextToast: ToastItem = {
+          id,
+          title: input.title,
+          body: input.body,
+          tone,
+          ttlMs,
+          action: input.action,
+          createdAt: now,
+        };
+
+        const withoutCurrent = prev.filter((toast) => toast.id !== id);
+        return [nextToast, ...withoutCurrent].slice(0, MAX_TOASTS);
+      });
+
+      const timeout = window.setTimeout(() => {
+        dismissToast(id);
+      }, ttlMs);
+      timersRef.current.set(id, timeout);
+      return id;
+    },
+    [clearTimer, dismissToast],
+  );
+
+  useEffect(() => () => {
+    for (const handle of timersRef.current.values()) {
+      window.clearTimeout(handle);
+    }
+    timersRef.current.clear();
+  }, []);
+
+  const value = useMemo<ToastContextValue>(
+    () => ({
+      toasts,
+      pushToast,
+      dismissToast,
+      clearToasts,
+    }),
+    [toasts, pushToast, dismissToast, clearToasts],
+  );
+
+  return <ToastContext.Provider value={value}>{children}</ToastContext.Provider>;
+}
+
+export function useToast() {
+  const context = useContext(ToastContext);
+  if (!context) {
+    throw new Error("useToast must be used within a ToastProvider");
+  }
+  return context;
+}