Streaming support for conversation history with SSE (#167)

* Streaming support with SSE

The main focus was on Gemini-CLI token streaming, which uses the standard JSON-Patch format to stream real-time updates to the frontend visa SSE.

There is also a default database-backed SSE implementation which covers the remaining executors like Claude-code.

* minor refactorings

frontend/package.json

@@ -15,6 +15,7 @@
   "dependencies": {
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/modifiers": "^9.0.0",
+    "@microsoft/fetch-event-source": "^2.0.1",
     "@radix-ui/react-dropdown-menu": "^2.1.15",
     "@radix-ui/react-label": "^2.1.7",
     "@radix-ui/react-portal": "^1.1.9",
@@ -28,6 +29,7 @@
     "class-variance-authority": "^0.7.0",
     "click-to-react-component": "^1.1.2",
     "clsx": "^2.0.0",
+    "fast-json-patch": "^3.1.1",
     "lucide-react": "^0.303.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",