loop: add diff stats from sketch-base to HEAD in /state endpoint

Add lines added/removed statistics computed from sketch-base to current HEAD,
displayed in webui Diff mode button for quick change overview.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s3f10ecf39df6b581k
diff --git a/loop/server/loophttp_test.go b/loop/server/loophttp_test.go
index 531f961..d93c360 100644
--- a/loop/server/loophttp_test.go
+++ b/loop/server/loophttp_test.go
@@ -262,6 +262,7 @@
 func (m *mockAgent) GetPortMonitor() *loop.PortMonitor { return loop.NewPortMonitor() }
 func (m *mockAgent) SkabandAddr() string               { return m.skabandAddr }
 func (m *mockAgent) LinkToGitHub() bool                { return false }
+func (m *mockAgent) DiffStats() (int, int)             { return 0, 0 }
 
 // TestSSEStream tests the SSE stream endpoint
 func TestSSEStream(t *testing.T) {