feat: multi-line edge labels

Connection labels now accept line breaks. The label field in EdgeModal
is a textarea (Enter inserts a newline) and the rendered edge label uses
whitespace-pre-line so newlines display as multiple lines.

Closes #183

ha-relevant: yes

frontend/src/components/canvas/edges/__tests__/HomelableEdge.label.test.tsx

@@ -0,0 +1,70 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render } from '@testing-library/react'
+import { ReactFlowProvider } from '@xyflow/react'
+import type { EdgeProps, Edge } from '@xyflow/react'
+import type { EdgeData } from '@/types'
+
+/**
+ * Issue #183 — connection labels must support multiple lines.
+ *
+ * The label is a free-text string; newlines entered in the EdgeModal textarea
+ * are stored verbatim. The rendered label div must preserve those newlines
+ * (`whitespace-pre-line`) instead of collapsing them into a single line.
+ *
+ * <EdgeLabelRenderer> normally portals into a node that only exists inside a
+ * full <ReactFlow> host, so we stub it to a passthrough to render the label
+ * markup directly.
+ */
+vi.mock('@xyflow/react', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@xyflow/react')>()
+  return {
+    ...actual,
+    EdgeLabelRenderer: ({ children }: { children: React.ReactNode }) => <>{children}</>,
+  }
+})
+
+const { HomelableEdge } = await import('../index')
+
+function renderEdge(data: Partial<EdgeData> = {}) {
+  const props = {
+    id: 'e1',
+    source: 'a',
+    target: 'b',
+    sourceX: 0,
+    sourceY: 0,
+    targetX: 100,
+    targetY: 100,
+    sourcePosition: 'bottom',
+    targetPosition: 'top',
+    data: { type: 'ethernet', ...data } as EdgeData,
+    selected: false,
+  } as unknown as EdgeProps<Edge<EdgeData>>
+
+  return render(
+    <ReactFlowProvider>
+      <svg>
+        <HomelableEdge {...props} />
+      </svg>
+    </ReactFlowProvider>,
+  )
+}
+
+describe('HomelableEdge label', () => {
+  it('renders the label text', () => {
+    const { getByText } = renderEdge({ label: 'uplink' })
+    expect(getByText('uplink')).toBeTruthy()
+  })
+
+  it('preserves newlines in the rendered label (issue #183)', () => {
+    const { container } = renderEdge({ label: 'line one\nline two' })
+    const label = Array.from(container.querySelectorAll('div.whitespace-pre-line')).find((d) =>
+      d.textContent === 'line one\nline two',
+    )
+    expect(label).toBeTruthy()
+  })
+
+  it('renders no label div when label is empty', () => {
+    const { container } = renderEdge({ label: undefined })
+    expect(container.querySelector('div.whitespace-pre-line')).toBeNull()
+  })
+})