feat: add chat components from jarvis frontend

- Migrated Chat.tsx with message handling and UI structure
- Migrated ChatInput.tsx with character limits and keyboard shortcuts
- Migrated MessageList.tsx with thinking/reasoning display
- Migrated ConversationSidebar.tsx (simplified placeholder)
- Migrated BackendStatusBanner.tsx (simplified placeholder)
- Created components/chat/index.ts barrel export
- Created app/chat/page.tsx placeholder route

These components are adapted from jarvis-fe but not yet fully functional:
- API calls placeholder (need to wire up /api/brain/query)
- Auth hooks stubbed (need useAuth implementation)
- Project/conversation hooks stubbed (need implementation)
- Imports changed from @jarvis/* to @mosaic/*

Next steps:
- Implement missing hooks (useAuth, useProjects, useConversations, useApi)
- Wire up backend API endpoints
- Add proper TypeScript types
- Implement full conversation management

apps/web/src/components/chat/BackendStatusBanner.tsx

@@ -0,0 +1,98 @@
+"use client";
+
+import { useState, useEffect } from "react";
+
+/**
+ * Banner that displays when the backend is unavailable.
+ * Shows error message, countdown to next retry, and manual retry button.
+ * 
+ * TODO: Integrate with actual backend status checking hook
+ */
+export function BackendStatusBanner() {
+  const [isAvailable, setIsAvailable] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+  const [retryIn, setRetryIn] = useState(0);
+
+  // TODO: Replace with actual useBackendStatus hook
+  // const { isAvailable, error, retryIn, manualRetry } = useBackendStatus();
+
+  const manualRetry = () => {
+    // TODO: Implement manual retry logic
+    console.log("Manual retry triggered");
+  };
+
+  const handleSignOut = async () => {
+    try {
+      // TODO: Implement signOut
+      // await signOut();
+    } catch (error) {
+      console.warn("Sign-out failed during backend unavailability:", error);
+    }
+    window.location.href = "/login";
+  };
+
+  // Don't render if backend is available
+  if (isAvailable) {
+    return null;
+  }
+
+  return (
+    <div
+      className="flex items-center justify-between gap-4 px-4 py-2 text-sm"
+      style={{
+        backgroundColor: "#fef3c7", // amber-100
+        borderBottom: "1px solid #fcd34d", // amber-300
+        color: "#92400e", // amber-800
+      }}
+      role="alert"
+      aria-live="polite"
+    >
+      <div className="flex items-center gap-2">
+        <svg
+          xmlns="http://www.w3.org/2000/svg"
+          className="h-5 w-5 flex-shrink-0"
+          viewBox="0 0 20 20"
+          fill="currentColor"
+          aria-hidden="true"
+        >
+          <path
+            fillRule="evenodd"
+            d="M8.257 3.099c.765-1.36 2.722-1.36 3.486 0l5.58 9.92c.75 1.334-.213 2.98-1.742 2.98H4.42c-1.53 0-2.493-1.646-1.743-2.98l5.58-9.92zM11 13a1 1 0 11-2 0 1 1 0 012 0zm-1-8a1 1 0 00-1 1v3a1 1 0 002 0V6a1 1 0 00-1-1z"
+            clipRule="evenodd"
+          />
+        </svg>
+        <span>
+          {error || "Backend temporarily unavailable."}
+          {retryIn > 0 && (
+            <span className="ml-1">
+              Retrying in {retryIn}s...
+            </span>
+          )}
+        </span>
+      </div>
+      <div className="flex items-center gap-2">
+        <button
+          onClick={manualRetry}
+          className="rounded px-3 py-1 text-xs font-medium transition-colors hover:opacity-80"
+          style={{
+            backgroundColor: "#fcd34d", // amber-300
+            color: "#92400e", // amber-800
+          }}
+        >
+          Retry Now
+        </button>
+        <button
+          onClick={handleSignOut}
+          className="rounded px-3 py-1 text-xs font-medium transition-colors hover:opacity-80"
+          style={{
+            backgroundColor: "transparent",
+            color: "#92400e", // amber-800
+            border: "1px solid #fcd34d", // amber-300
+          }}
+        >
+          Sign in again
+        </button>
+      </div>
+    </div>
+  );
+}