feat(frontend): add resizable split panel layout to LTT page (sub-phase 2.4)

Replace fixed CSS Grid with react-resizable-panels v4 (Group/Panel/Separator).
Upper panel (video + query) defaults to 30%, lower panel (response) to 70%.
Draggable divider with hover/active state via data-separator attributes.
Add ResizeObserver and DOMRect polyfills to test setup for jsdom compatibility.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/package-lock.json

@@ -15,6 +15,7 @@
         "react": "^18.2.0",
         "react-dom": "^18.2.0",
         "react-markdown": "^10.1.0",
+        "react-resizable-panels": "^4.10.0",
         "react-router-dom": "^7.14.2",
         "tailwindcss": "^3.4.0"
       },
@@ -4959,6 +4960,16 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/react-resizable-panels": {
+      "version": "4.10.0",
+      "resolved": "https://registry.npmjs.org/react-resizable-panels/-/react-resizable-panels-4.10.0.tgz",
+      "integrity": "sha512-frjewRQt7TCv/vCH1pJfjZ7RxAhr5pKuqVQtVgzFq/vherxBFOWyC3xMbryx5Ti2wylViGUFc93Etg4rB3E0UA==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^18.0.0 || ^19.0.0",
+        "react-dom": "^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/react-router": {
       "version": "7.14.2",
       "resolved": "https://registry.npmjs.org/react-router/-/react-router-7.14.2.tgz",

frontend/package.json

@@ -17,6 +17,7 @@
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-markdown": "^10.1.0",
+    "react-resizable-panels": "^4.10.0",
     "react-router-dom": "^7.14.2",
     "tailwindcss": "^3.4.0"
   },

frontend/src/pages/LTTPage.tsx

@@ -1,5 +1,6 @@
 import React from 'react'
 import { Film } from 'lucide-react'
+import { Group, Panel, Separator } from 'react-resizable-panels'
 import { useQueryDocument } from '../lib/queries'
 import { QueryInput } from '../components/QueryInput'
 import { ExtractedQuestionsDisplay } from '../components/ExtractedQuestionsDisplay'
@@ -24,22 +25,35 @@ export const LTTPage: React.FC = () => {
   }
 
   return (
-    <div className="h-full grid grid-rows-[30%_1fr] grid-cols-2 bg-gray-50">
-      <div className="border-r border-b border-gray-200 p-4 min-h-0 overflow-hidden">
-        <VideoPlaceholder />
-      </div>
-      <div className="border-b border-gray-200 p-6 flex flex-col gap-4 overflow-y-auto min-h-0">
-        <QueryInput onSubmit={handleQuerySubmit} isLoading={queryMutation.isPending} />
-        <ExtractedQuestionsDisplay extractedQuestions={queryMutation.data?.extracted_questions} isLoading={queryMutation.isPending} />
-      </div>
-      <div className="col-span-2 p-6 border-t border-gray-200 overflow-y-auto min-h-0">
-        <ResponsePanel
-          answer={queryMutation.data?.answer ?? null}
-          sources={queryMutation.data?.sources ?? []}
-          isLoading={queryMutation.isPending}
-          error={queryMutation.isError ? (queryMutation.error instanceof Error ? queryMutation.error.message : 'Query failed') : null}
-        />
-      </div>
+    <div className="h-full bg-gray-50">
+      <Group orientation="vertical" id="ltt-main-group">
+        <Panel id="ltt-upper-panel" defaultSize={30} minSize={15} maxSize={60}>
+          <div className="h-full grid grid-cols-2">
+            <div className="border-r border-gray-200 p-4 min-h-0 overflow-hidden">
+              <VideoPlaceholder />
+            </div>
+            <div className="p-6 flex flex-col gap-4 overflow-y-auto min-h-0">
+              <QueryInput onSubmit={handleQuerySubmit} isLoading={queryMutation.isPending} />
+              <ExtractedQuestionsDisplay extractedQuestions={queryMutation.data?.extracted_questions} isLoading={queryMutation.isPending} />
+            </div>
+          </div>
+        </Panel>
+
+        <Separator className="h-2 cursor-row-resize flex items-center justify-center bg-gray-200 [&[data-separator='hover']]:bg-blue-300 [&[data-separator='active']]:bg-blue-400 transition-colors">
+          <div className="w-8 h-1 rounded-full bg-gray-400 [&[data-separator='hover']_&]:bg-blue-500 [&[data-separator='active']_&]:bg-blue-600 transition-colors" />
+        </Separator>
+
+        <Panel id="ltt-lower-panel" minSize={20}>
+          <div className="h-full p-6 border-t border-gray-200 overflow-y-auto">
+            <ResponsePanel
+              answer={queryMutation.data?.answer ?? null}
+              sources={queryMutation.data?.sources ?? []}
+              isLoading={queryMutation.isPending}
+              error={queryMutation.isError ? (queryMutation.error instanceof Error ? queryMutation.error.message : 'Query failed') : null}
+            />
+          </div>
+        </Panel>
+      </Group>
     </div>
   )
 }

frontend/src/test/setup.ts

@@ -1 +1,12 @@
 import '@testing-library/jest-dom'
+
+global.ResizeObserver = class ResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+
+global.DOMRect = class DOMRect {
+  x = 0; y = 0; width = 0; height = 0; top = 0; right = 0; bottom = 0; left = 0
+  static fromRect() { return new DOMRect() }
+}