refactor(web): enhance empty state in RepositoryAlgorithmsPanel

Match the improved empty state from GraphAlgorithmsPanel with:
- "Build Your Graph" heading
- Clear call-to-action guidance
- Specific instruction about using the graph icon button

Author: Mearman

apps/web/src/components/algorithms/RepositoryAlgorithmsPanel.tsx

@@ -131,15 +131,19 @@ export const RepositoryAlgorithmsPanel = () => {
 
   if (!hasData) {
     return (
-      <Card style={{ border: BORDER_STYLE_GRAY_3 }} p="md">
-        <Alert icon={<IconAlertCircle size={ICON_SIZE.MD} />} title="No Graph Data" color="gray">
-          <Text size="sm">
-            Add entities to the repository to analyze their relationships using graph algorithms.
-          </Text>
-          <Text size="xs" c="dimmed" mt="xs">
-            Browse works, authors, or institutions and add them to the repository to build a graph.
-          </Text>
-        </Alert>
+      <Card style={{ border: BORDER_STYLE_GRAY_3 }} p="lg">
+        <Stack align="center" gap="md" mih="200px" justify="center">
+          <IconGraph size={ICON_SIZE.XL} color="var(--mantine-color-gray-4)" style={{ opacity: 0.5 }} />
+          <Stack gap="xs" ta="center">
+            <Title order={4} c="dimmed">Build Your Graph</Title>
+            <Text size="sm" c="dimmed">
+              Add entities from search results or entity detail pages to start analyzing connections.
+            </Text>
+            <Text size="xs" c="dimmed" mt="sm">
+              Use the graph icon button on any entity to add it here.
+            </Text>
+          </Stack>
+        </Stack>
       </Card>
     );
   }