Add bridge lane health API and config-ready lane UI for Tier A Week 3.

Probe LINK balances on CCIP bridge contracts, expose proof-transfer metadata on bridge status, and render funded/unfunded lane health on /bridge with extended smoke coverage.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/api/track1/bridge_mode_test.go

@@ -30,6 +30,14 @@ func TestResolveBridgeDeliveryModeMixedWhenTransactionVisibilityStale(t *testing
 	require.Equal(t, "bridge_monitoring_and_homepage", got.Scope)
 }
 
+func TestResolveBridgeDeliveryModeMixedWhenQuietChain(t *testing.T) {
+	diagnostics := &freshness.Diagnostics{
+		ActivityState: "quiet_chain",
+	}
+	got := resolveBridgeDeliveryMode(false, diagnostics, freshness.CompletenessComplete)
+	require.Equal(t, "live", got.Kind)
+}
+
 func TestIsStaleTransactionVisibility(t *testing.T) {
 	require.True(t, isStaleTransactionVisibility(&freshness.Diagnostics{ActivityState: "fresh_head_stale_transaction_visibility"}))
 	require.False(t, isStaleTransactionVisibility(&freshness.Diagnostics{ActivityState: "healthy"}))