Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/components/fileManager/DesktopLayout.tsx

@@ -0,0 +1,98 @@
+import React from 'react';
+import { Grid } from '@mantine/core';
+import FileSourceButtons from '@app/components/fileManager/FileSourceButtons';
+import FileDetails from '@app/components/fileManager/FileDetails';
+import SearchInput from '@app/components/fileManager/SearchInput';
+import FileListArea from '@app/components/fileManager/FileListArea';
+import FileActions from '@app/components/fileManager/FileActions';
+import HiddenFileInput from '@app/components/fileManager/HiddenFileInput';
+import { useFileManagerContext } from '@app/contexts/FileManagerContext';
+
+const DesktopLayout: React.FC = () => {
+  const {
+    activeSource,
+    recentFiles,
+    modalHeight,
+  } = useFileManagerContext();
+
+  return (
+    <Grid gutter="xs" h="100%" grow={false} style={{ flexWrap: 'nowrap', minWidth: 0 }}>
+      {/* Column 1: File Sources */}
+      <Grid.Col span="content" p="lg" style={{
+        minWidth: '13.625rem',
+        width: '13.625rem',
+        flexShrink: 0,
+        height: '100%',
+      }} data-tour="file-sources">
+        <FileSourceButtons />
+      </Grid.Col>
+
+      {/* Column 2: File List */}
+      <Grid.Col span="auto" style={{
+        display: 'flex',
+        flexDirection: 'column',
+        height: '100%',
+        minHeight: 0,
+        minWidth: 0,
+        flex: '1 1 0px'
+      }}>
+        <div style={{
+          flex: 1,
+          display: 'flex',
+          flexDirection: 'column',
+          backgroundColor: 'var(--bg-file-list)',
+          border: '1px solid var(--mantine-color-gray-2)',
+          boxShadow: '0 2px 8px rgba(0, 0, 0, 0.1)',
+          overflow: 'hidden'
+        }}>
+          {activeSource === 'recent' && (
+            <>
+              <div style={{
+                flexShrink: 0,
+                borderBottom: '1px solid var(--mantine-color-gray-3)'
+              }}>
+                <SearchInput />
+              </div>
+              <div style={{
+                flexShrink: 0,
+                borderBottom: '1px solid var(--mantine-color-gray-3)'
+              }}>
+                <FileActions />
+              </div>
+            </>
+          )}
+
+          <div style={{ flex: 1, minHeight: 0 }}>
+            <FileListArea
+              scrollAreaHeight={`calc(${modalHeight} )`}
+              scrollAreaStyle={{
+                height: activeSource === 'recent' && recentFiles.length > 0 ? modalHeight : '100%',
+                backgroundColor: 'transparent',
+                border: 'none',
+                borderRadius: 0
+              }}
+            />
+          </div>
+        </div>
+      </Grid.Col>
+
+      {/* Column 3: File Details */}
+      <Grid.Col p="xl" span="content" style={{
+        minWidth: '25rem',
+        width: '25rem',
+        flexShrink: 0,
+        height: '100%',
+        maxWidth: '18rem'
+      }}>
+        <div style={{ height: '100%', overflow: 'hidden' }}>
+          <FileDetails />
+        </div>
+      </Grid.Col>
+
+      {/* Hidden file input for local file selection */}
+      <HiddenFileInput />
+    </Grid>
+  );
+};
+
+export default DesktopLayout;