refactor: replace hand-rolled SSE with Hono + fetch-event-source

Server (index.ts):
- migrate to Hono streamSSE, mounted under Bun.serve fetch handler
- idleTimeout: 255 fixes the silent Bun 10s timeout that killed SSE
  responses before the first keepalive could fire (root cause of the
  empty EventStream tab)
- stream.onAbort wires an AbortController into upstream fetch signal
- 15s : keepalive raw SSE comments for Cloudflare 120s headroom
- decodeDataUrl returns Uint8Array<ArrayBuffer> for DOM Blob types
- chromeDevToolsAutomaticWorkspaceFolders: false silences the
  'Unable to add filesystem' warning in sandboxed browsers

Client (client.ts new):
- extracted from inline <script> — Bun only bundles external script src,
  not inline module imports, so node_modules bare specifiers must live
  in their own file
- @microsoft/fetch-event-source replaces hand-rolled fetch +
  ReadableStream parsing; supports POST + body + signal natively
- client aborts the loop on event:done so fetchEventSource doesn't retry

Build:
- drop unused react/react-dom/@types/react* deps (KISS)
- add 'DOM', 'DOM.Iterable' to tsconfig lib for client.ts

bun.lock

@@ -5,35 +5,25 @@
     "": {
       "name": "ai-playground",
       "dependencies": {
-        "react": "^19.2.6",
-        "react-dom": "^19.2.6",
+        "@microsoft/fetch-event-source": "^2.0.1",
+        "hono": "^4.12.19",
       },
       "devDependencies": {
         "@types/bun": "^1.3.14",
-        "@types/react": "^19.2.14",
-        "@types/react-dom": "^19.2.3",
         "typescript": "^6.0.3",
       },
     },
   },
   "packages": {
+    "@microsoft/fetch-event-source": ["@microsoft/fetch-event-source@2.0.1", "", {}, "sha512-W6CLUJ2eBMw3Rec70qrsEW0jOm/3twwJv21mrmj2yORiaVmVYGS4sSS5yUwvQc1ZlDLYGPnClVWmUUMagKNsfA=="],
+
     "@types/bun": ["@types/bun@1.3.14", "", { "dependencies": { "bun-types": "1.3.14" } }, "sha512-h1hFqFVcvAvD9j9K7ZW7vd82aSA+rTdznZa+5bwvCwqSB1jmmfLcbIWhOLx1/+boy/xmjgCs/OMUL8hRJSmnPw=="],
 
     "@types/node": ["@types/node@25.9.0", "", { "dependencies": { "undici-types": ">=7.24.0 <7.24.7" } }, "sha512-AOQwYUNolgy3VosiRqXrACUXTN8nJUtPl7FJXMqZVyxiiCLhQuG3jXKvCS1ALr+Y2OmZhzzLVlYPEqJaiqkaJQ=="],
 
-    "@types/react": ["@types/react@19.2.14", "", { "dependencies": { "csstype": "^3.2.2" } }, "sha512-ilcTH/UniCkMdtexkoCN0bI7pMcJDvmQFPvuPvmEaYA/NSfFTAgdUSLAoVjaRJm7+6PvcM+q1zYOwS4wTYMF9w=="],
-
-    "@types/react-dom": ["@types/react-dom@19.2.3", "", { "peerDependencies": { "@types/react": "^19.2.0" } }, "sha512-jp2L/eY6fn+KgVVQAOqYItbF0VY/YApe5Mz2F0aykSO8gx31bYCZyvSeYxCHKvzHG5eZjc+zyaS5BrBWya2+kQ=="],
-
     "bun-types": ["bun-types@1.3.14", "", { "dependencies": { "@types/node": "*" } }, "sha512-4N0ig0fEomHt5R0KCFWjovxow98rIoRwKolrYdCcknNwMekCXRnWEUvgu5soYV8QXtVsrUD8B95MBOZGPvr6KQ=="],
 
-    "csstype": ["csstype@3.2.3", "", {}, "sha512-z1HGKcYy2xA8AGQfwrn0PAy+PB7X/GSj3UVJW9qKyn43xWa+gl5nXmU4qqLMRzWVLFC8KusUX8T/0kCiOYpAIQ=="],
-
-    "react": ["react@19.2.6", "", {}, "sha512-sfWGGfavi0xr8Pg0sVsyHMAOziVYKgPLNrS7ig+ivMNb3wbCBw3KxtflsGBAwD3gYQlE/AEZsTLgToRrSCjb0Q=="],
-
-    "react-dom": ["react-dom@19.2.6", "", { "dependencies": { "scheduler": "^0.27.0" }, "peerDependencies": { "react": "^19.2.6" } }, "sha512-0prMI+hvBbPjsWnxDLxlCGyM8PN6UuWjEUCYmZhO67xIV9Xasa/r/vDnq+Xyq4Lo27g8QSbO5YzARu0D1Sps3g=="],
-
-    "scheduler": ["scheduler@0.27.0", "", {}, "sha512-eNv+WrVbKu1f3vbYJT/xtiF5syA5HPIMtf9IgY/nKg0sWqzAUEvqY/xm7OcZc/qafLx/iO9FgOmeSAp4v5ti/Q=="],
+    "hono": ["hono@4.12.19", "", {}, "sha512-xa3eYXYXx68XTT4hZ7dRzsXBhaq85ToSrlUJNoR0gwz/1Ap/CNwX47wfvV7pc/xWhjKVVkLT7zBJy8chhNguqQ=="],
 
     "typescript": ["typescript@6.0.3", "", { "bin": { "tsc": "bin/tsc", "tsserver": "bin/tsserver" } }, "sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw=="],