feat(08-01): add diff viewer component and API client

- Add DiffResponse interface and getDiff method to configHistoryApi
- Create DiffViewer component with unified diff rendering
- Green highlighting for added lines, red for removed lines
- Blue styling for hunk headers, loading skeleton, error state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -981,6 +981,16 @@ export interface ConfigChangeEntry {
   snapshot_id: string
 }
 
+export interface DiffResponse {
+  id: string
+  diff_text: string
+  lines_added: number
+  lines_removed: number
+  old_snapshot_id: string
+  new_snapshot_id: string
+  created_at: string
+}
+
 export const configHistoryApi = {
   list: (tenantId: string, deviceId: string, limit = 50, offset = 0) =>
     api
@@ -989,6 +999,13 @@ export const configHistoryApi = {
         { params: { limit, offset } },
       )
       .then((r) => r.data),
+
+  getDiff: (tenantId: string, deviceId: string, snapshotId: string) =>
+    api
+      .get<DiffResponse>(
+        `/api/tenants/${tenantId}/devices/${deviceId}/config/${snapshotId}/diff`,
+      )
+      .then((r) => r.data),
 }
 
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