feat(dashboard): add toast notification system with error tracking

Add lightweight toast notification infrastructure that surfaces
repeated errors to users while avoiding alert fatigue.

Components:
- ToastContainer: Renders toast notifications with auto-dismiss
- showToast(): Singleton function to display messages from anywhere
- trackError(): Counts errors by key, surfaces toast after threshold

Error tracking strategy:
- Errors logged immediately (console.error)
- Toast shown only after 3+ occurrences within 30-second window
- Prevents spam from transient network issues
- Reset counter on success (clearErrorCount)

Toast styling:
- Fixed position bottom-right with z-index 100
- Type-aware colors (error=red, success=green, info=yellow)
- Manual dismiss button with auto-dismiss after 5 seconds
- Backdrop blur and slide-up animation

This prepares the dashboard to gracefully handle API failures
without overwhelming users with error popups for transient issues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dashboard/components/Toast.js

@@ -0,0 +1,125 @@
+import { html, useState, useEffect, useCallback, useRef } from '../lib/preact.js';
+
+/**
+ * Lightweight toast notification system.
+ * Tracks error counts and surfaces persistent issues.
+ */
+
+// Singleton state for toast management (shared across components)
+let toastListeners = [];
+let toastIdCounter = 0;
+
+export function showToast(message, type = 'error', duration = 5000) {
+  const id = ++toastIdCounter;
+  const toast = { id, message, type, duration };
+  toastListeners.forEach(listener => listener(toast));
+  return id;
+}
+
+export function ToastContainer() {
+  const [toasts, setToasts] = useState([]);
+  const timeoutIds = useRef(new Map());
+
+  useEffect(() => {
+    const listener = (toast) => {
+      setToasts(prev => [...prev, toast]);
+      if (toast.duration > 0) {
+        const timeoutId = setTimeout(() => {
+          timeoutIds.current.delete(toast.id);
+          setToasts(prev => prev.filter(t => t.id !== toast.id));
+        }, toast.duration);
+        timeoutIds.current.set(toast.id, timeoutId);
+      }
+    };
+    toastListeners.push(listener);
+    return () => {
+      toastListeners = toastListeners.filter(l => l !== listener);
+      // Clear all pending timeouts on unmount
+      timeoutIds.current.forEach(id => clearTimeout(id));
+      timeoutIds.current.clear();
+    };
+  }, []);
+
+  const dismiss = useCallback((id) => {
+    // Clear auto-dismiss timeout if exists
+    const timeoutId = timeoutIds.current.get(id);
+    if (timeoutId) {
+      clearTimeout(timeoutId);
+      timeoutIds.current.delete(id);
+    }
+    setToasts(prev => prev.filter(t => t.id !== id));
+  }, []);
+
+  if (toasts.length === 0) return null;
+
+  return html`
+    <div class="fixed bottom-4 right-4 z-[100] flex flex-col gap-2 pointer-events-none">
+      ${toasts.map(toast => html`
+        <div
+          key=${toast.id}
+          class="pointer-events-auto flex items-start gap-3 rounded-xl border px-4 py-3 shadow-lg backdrop-blur-sm animate-fade-in-up ${
+            toast.type === 'error'
+              ? 'border-attention/50 bg-attention/15 text-attention'
+              : toast.type === 'success'
+              ? 'border-active/50 bg-active/15 text-active'
+              : 'border-starting/50 bg-starting/15 text-starting'
+          }"
+          style=${{ maxWidth: '380px' }}
+        >
+          <div class="flex-1 text-sm font-medium">${toast.message}</div>
+          <button
+            onClick=${() => dismiss(toast.id)}
+            class="shrink-0 rounded p-0.5 opacity-70 transition-opacity hover:opacity-100"
+          >
+            <svg class="h-4 w-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/>
+            </svg>
+          </button>
+        </div>
+      `)}
+    </div>
+  `;
+}
+
+/**
+ * Error tracker for surfacing repeated failures.
+ * Tracks errors by key and shows toast after threshold.
+ */
+const errorCounts = {};
+const ERROR_THRESHOLD = 3;
+const ERROR_WINDOW_MS = 30000; // 30 second window
+
+export function trackError(key, message, { log = true, threshold = ERROR_THRESHOLD } = {}) {
+  const now = Date.now();
+
+  // Always log
+  if (log) {
+    console.error(`[${key}]`, message);
+  }
+
+  // Track error count within window
+  if (!errorCounts[key]) {
+    errorCounts[key] = { count: 0, firstAt: now, lastToastAt: 0 };
+  }
+
+  const tracker = errorCounts[key];
+
+  // Reset if outside window
+  if (now - tracker.firstAt > ERROR_WINDOW_MS) {
+    tracker.count = 0;
+    tracker.firstAt = now;
+  }
+
+  tracker.count++;
+
+  // Surface toast after threshold, but not too frequently
+  if (tracker.count >= threshold && now - tracker.lastToastAt > ERROR_WINDOW_MS) {
+    showToast(`${message} (repeated ${tracker.count}x)`, 'error');
+    tracker.lastToastAt = now;
+    tracker.count = 0; // Reset after showing toast
+  }
+}
+
+export function clearErrorCount(key) {
+  delete errorCounts[key];
+}